The 921 LED Reverse Light Bulb Study

In an effort to help inform people on how fundamentally different retrofitting LED lighting technology into incandescent housings is (vs upgrading the stock incandescent source), I decided to put together a small study on several different LED designs used for the 921 reverse light bulb housing to look at how their differing designs affect their performance in the application.

This thread is intended for educational purposes.



The 921 reverse light bulb is used in 2001-2017+ Tacomas as well as many other vehicles.

Background:
A challenge for LEDs is the design of the bulb in relation to the housing it is placed in due to the directional nature of LED light. For an incandescent light source like the 921 housing is designed for, a brighter incandescent light directly equates to more light output our of the reverse light assembly. Whereas for an LED light source, brighter or more powerful does not necessarily translate directly into more light output, so it depends on where/how the manufacturer has oriented the diodes on the bulb assembly. Examples: Does it make use of the reflector lens, or face directly rear bypassing the reflector, or a combination of both? Are many small diodes used running lower power pointing in many directions, or few larger diodes used running higher power pointing in fewer directions. These variables don't exist in an incandescent bulb design, and they have a direct impact on the effectiveness of the LED bulb. So while an LED bulb can be brighter than an incandescent based bulb, or one LED can be brighter than another, that does not necessarily mean it will perform better in the application.

But wait! You shouldn't run LEDs in reflectors!
I've previously documented why you should not run LEDs in your headlight low beam/fog light reflector housings in this thread. However, those are forward facing lights critical for safety with engineered cut offs horizons to avoid blinding on coming drivers, and minimize reflection back to the driver from precipitation in poor weather conditions. Reverse lights have no cut off horizon and emit light in all directions, therefore glare is not an issue like it is for forward facing lighting.

The Bulbs
I deliberately used bulbs that differed in design to see which bulb design did best.

OEM 921 Incandescent (baseline control point)
Published specs: 264 lumens, 18w, 2800k Cost: $5/pair Warranty: 1yr


These are the OEM incandescent bulbs. These true incandescent bulbs (vs a halogen bulb) are the oldest and least efficient form of lighting technology. Their primary advantage against LEDs is the omnidirectional light output that the 921 reverse light housing is designed for.


Bulbbeats/Flashwolves 921 LED
Published specs: 1000 lumens, 4w, 6000k Cost: $13/pair Warranty: N/A


A blade type design with many small diodes that focuses the majority of light output to the sides of the lighting assembly, which would indirectly light the reflector, with some diodes to pointing straight out the rear of the lens. I purchased these based on them being popular on the forum. The lumen output claims are quite outlandish compared to other 921 LEDs from respectable manufacturers, especially considering the price point. You know how the saying goes.


Philips Vision 921 LED
Publish specs: 200 lumens, 3.4w, 6000k Cost: $40/pair Warranty: 12 years


Philips has an interesting inverted design where the only 2 LEDs actually directly face the reflector inside a bucket, so 100% of the light output is bounced off the reflector. I figured these would have the most OEM looking beam pattern by exclusively using the reflector. These were the only lights to be labelled as 'street legal' directly on the box. Note their specs for lumen output are actually lower than the stock OEM bulbs.

$23 SYLVANIA - 921 T-16 W16W ZEVO LED White Bulb - Bright LED Bulb, Ideal for Interior Lighting - Map, Dome, Trunk, Cargo and License Plate (Contains 2 Bulbs)

Sylvania Zero 921 LED
Published specs: N/A lumens, 2.5w, 6000k Cost: $22/pair Warranty: Lifetime


The Sylvanias are the type of over the counter LED bulb usually found at your local autoparts store, though labeled on the box 'For off road use only'. These have 6 small diodes in a circularpattern facing away from the reflector, angled at roughly 45 degrees. By omitting the reflector from the design and using small diodes while also being the lowest power bulb of the group, optimal performance does not seem likely.


Morimoto XB 921 LEDs
Published specs: 350 lumens, 2.6w (185mA), 5500k Cost $25/pair Warranty: 2 years


Definitely the most exotic looking bulb design with a respectable lumen spec. The bulb features a single massive Cree LED in an aluminum housing with a glass lens. These feel very high quality. This design will also make no use of the reflector, and with the directional design of a single LED in a tube, it should produce a different beam pattern than stock.


DeAutoLED 921 LEDs
Published specs: N/A, 5500k Cost $50/pair Warranty: Lifetime


This is another blade type design similar to the bulbbeats/flashwolves, but with fewer though larger diodes. This design will also make use of the reflector with indirect light while pointing diodes directly out the rear of the lens. While this and the bulbbeats look similar, the weight of the DeAutoLED bulb feels heavier.
*Full disclosure: These bulbs were provided by DeAutoLED for inclusion in this study.


Philips Gy6.35 HIR
Published specs: 750 lumens, 30w, 3000k Cost: $27/pair Warranty: 1yr


The Gy6.35 is a 12v landscape bulb that can be inserted into the 921 socket. HIR stands for Halogen InfraRed, which uses a IR reflective coating on the halogen bulb to direct IR energy back to the filament causing it to burn significantly brighter than a standard halogen bulb. This is the most advanced form of incandescent technology. The bulb does not lock into the socket like a stock bulb, so the prongs must be bent to create tension to keep it there which causes some to be dissatisfied with this solution. While HIRs are not the focus of this study, comparison questions are sure to be asked based on my HIR reverse light thread so I included it as a comparison to one of the highest performing LEDs.


Some notes on testing lighting
When taking photos of light output, different automatic aperture and focus settings on a camera can produce very different looking images of the same lights. This is especially true facing the light source. It is easily possible for a lower light level to look brighter than a higher light level, as the camera compensates to make the photo less dark or not overexposed. So taking output photos of both bulbs swapped and then comparing it to output photos of a different set swapped is completely useless for light output comparison. The best way to take light output photos without using a full manual control SLR, is to compare the bulbs side by side in the same photo. That way all the photo variables are the same giving a true accurate representation of the light output comparison. As a control point in this study, all LED bulbs are compared in photos to the stock incandescent OEM bulb. Though I did my best to control the focus point to make the pictures consistent using an iphone7 with a 12MP camera, bulb output picture comparisons between photos should be based off how well the bulb did compared to the stock incandescent bulb control point.

I've been challenged on my negative stance of using GIFs (which are essentially multiple overlapped low quality images to play like a slideshow/video) or multiple images of both lights swapped to compare light output. I do not believe these types of image comparisons are good for light output analysis.

Let me provide an example of why this type of comparison is unjust.

Light set 1


Light set 2


Looking at these images, Light set 2 is the clear obvious winner for output. However, both these photos are actually the exact same lights, with photos taken under the exact same conditions, with the same camera, from the same position, within seconds from each other. The only difference is aperture setting from the camera looking ever so slightly in one direction vs another for focus causing drastic differences in perceived output. It is for this reason that I use side-by-side comparison photos in my reviews to eliminate camera bias as much as practical.

Something that is often overlooked is voltage. Bulbs are brighter at higher voltage, especially incandescent bulbs. If you run the test with the vehicle off, you are running at lower voltage and significantly less light output than with the vehicle on when voltage is being provided by the alternator. If comparing an LED vs incandescent, having the vehicle off will heavily penalize the incandescent output. So it is important that all light test are done with the vehicle running to get the full voltage provided by the alternator. Running the vehicle will also provide a more realistic use case.

I've recently learned that not all lux meters are ideal for testing LED specific light output due to how they are calibrated. As such, for this LED study, I upgraded my $40 hobbyist grade lux meter to a commercial grade $300 LED calibrated multi-light spectrum meter, designed specifically to accurately measure LED light output. The new meter is the Extech LT45, which also still accurately measures incandescent sources as well. I chose this meter as it is capable of full color spectrum LED analysis, vs lesser LED models that only do 'white' LED readings.

While it is great to have quantitative results with a lux meter, it is just one tool for analyzing output, it does not tell the whole story. A lux reading from a bulb in a lighting assembly will tell you how bright the output is in a specific point in the beam. It does not tell you cumulative output. This is especially true with the directional light nature of LEDs, as their light output has more hot spots than omnidirectional incandescent bulbs.


The Tests
Each LED was compared side-by-side to the stock OEM 921 incandescent bulb as a control point. There were 5 tests run on each LED bulb.

1) Light output as seen from the bed. This is the most valuable in my opinion is it best represents how the light output will perform and be seen from inside the truck. To better compare output distribution I built 4 small white markers and placed 2 behind the truck just to the outside of the body as would be seen in a side mirror, and 2 to the far edge of what I could squeeze in a photo to show the extent of the light distribution. The 2 outside markers help show how well the proximity is illuminated if you wanted to back up and make a turn. The markers were measured to be equal distance from the truck and from the centerline to their companion marker.

2) Light output facing the truck. This is a different way of looking at the light output and mainly allows one to see the aesthetics of the bulb in use for those that are interested. This is the most difficult to have consistent photos and least useful for output comparison as different light output beams and intensity directly affect the camera more than any other kind of output photos. A light source that has more glare or directional intensity (which is different than output), like many LEDs, will often appear brighter when facing the light source than a light with a more consistent full uniform pattern which may actually put out more light. To see an example of how poor a test this is for judging output, look at the output pics facing the light source in this thread and compare them to the output pics at the end of the thread.

3) Wall shots. This helps understand beam pattern and light distribution and to some extent intensity. These photos are useful to compare the beam pattern to stock. -- While I did take these the LEDs actually all look about the same in wall shot the photos, so I have chosen to exclude them as they don't add value.

4) Lux readings. Quantitative measured output of the bulb in the light assembly to remove subjectivity. Many people confuse color with output, thinking whiter/bluer light must be brighter, lux readings help put things in perspective. However, lux readings don’t tell the complete story, as mentioned in my notes in the light testing section above.

5) Light spectrum reading. Seeing which wavelengths are emitted by the bulb, as broader color spectrum is better for vision. This test is more just for casual interest.
--This was actually challenging to perform and I just ran it on a single LED vs HIR for comparison as the outcome of LEDs vs incandescents should be similar across the board.

To standardized the photo tests as much as possible, I invested in a few tripods so that each photo position and angle is identical to the next for the most optimal comparisons.

Results

First, OEM 921 incandescent
lux: 1439



Stock vs Morimoto XB
Lux: 1709






The Morimoto XB is incredibly bright to the point of being hard to look at from behind the truck, however the output is extremely focused and directionalized with the LED being in a tube. The side marker in the test is hardly illuminated and the peripheral area on the driver side is not illuminated. Note how you can barely make out the corner of the bed or edge of the manhole cover. While these do an excellent job of illuminating what is directly behind you, they don't do a good job illuminating the entire area so that you are aware of your surroundings. The design looks like it could be improved by moving the LED closer to the lens in the tube to achieve greater light dispersion.

Stock vs Flashwolves
Lux: 1094






The Flashwolves with the blade design did a good job with light distribution, however their light output seems pretty on par with stock (264 lumen), a far cry from the claimed 1000 lumens. I would consider these to be a cosmetic switch for those wanting to inexpensively change light color only as there is no performance improvement.


Stock vs Sylvania Zevo
Lux: Sylvania: 769







The Sylvania Zevos actually do surprisingly better than their lux rating and wattage would have you guess. The bulb provides excellent near field ground illumination. Looking at the bulb design, it has 3 of the 6 diodes pointing down to the ground at roughly 45 degrees around the ball design. It is the only LED in the group that actually points the diodes in the bulb at the ground. While this clearly helps near field illumination, I think you would lose some distance illumination which is usually less of a concern for reverse usage.


Stock vs Philips
Lux: Philips: 1583






The Philips LEDs beam was not as consistent with stock beam pattern as I was expecting. It did well in mid-far distance but was not as bright immediately behind/around the truck. The Philips bounce 100% of the light off the reflector, while this seems like a good idea, I believe it creates less light immediately behind the truck as the stock incandescent would have light exit from the bulb in all directions including immediately toward the ground for near field illumination, in addition to utilizing the reflector for mid-far illumination, whereas the Philips loses that ability to have the light exit immediately toward the ground.


Stock vs DeAutoLED
Lux 1633






The DeAutoLED bulbs do very well providing even light distribution with the blade design, using side firing and outward firing diodes like the flashwolves. However unlike the flashwolves they are noticeably brighter than stock. These seem to have the best light distribution paired with performance out of the LED group.


Morimoto XB (passenger) vs DeAutoLED (driver)




Comparing the 2 highest output LEDs, Morimotos and the DeAutoLED you can see that the Morimotos are clearly brighter, but the light is not well distributed with the projector style design vs the blade type design. Note how the passenger side marker is not illuminated. This is very apparent when you look at the wide angle reverse camera below and cannot see what is behind you to the passenger side with the Morimotos as the screen is just dark.




Philips HIR vs DeAutoLED
HIR Lux 4330




HIR lux:


The HIRs are so bright the Lux meter has automatically adjusted the scale by 10x, for a reading of 4330 lux. vs LEDs in the mid-1xxx range. This is not surprising, as the Philips HIRs are rated at 750 lumens, which is over 2x the brightest LED. And unlike the next closest LED, they are an omni-directional light source, so not only do they get the benefit of direct light, but can also make full use of the reflector without compromise. As an omnidirectional light source the Philips HIR provides excellent uniform light distribution.
Note: The HIRs are not 100% true plug and play like the LEDs. For more info see the Gy6.35 HIR thread.

You can tell the light source calibration in the Lux meter by the reading at the bottom of the display, L.S. L0 = Incandescent. L.S. L1 = White LED.

Notes on accuracy of lux testing
As I mentioned earlier in the light testing notes, lux does not tell the complete story. However it is a useful tool to have in the light testing arsenal. Every good engineer should always question the data.

For those that are unfamiliar, lux is a measurement of lumens/area, meaning there is a direct correlation with lumen output and lux readings. These readings work well in uniform light, but can be more challenging in non-uniform directionalized light (LEDs).

For due diligence of curiosity I'll do a little napkin math.
The OEM bulbs have a lumen rating of 264. Philips (a widely respected manufacture) HIRs have a published lumen rating of 750. Meaning on the spec sheet the HIRs are 2.85 brighter than stock.

I took a lux reading of 1439 for the stock bulbs. To be inline with the published specs the HIRs should be 2.85x brighter, or a lux reading of 4101. The lux meter has an accuracy error of +/-3%, meaning it could be up to a lux reading of 4224. My measured lux reading was 4330, though I lost a significant digit due to 10x scaling because of the higher output. Which translates to the readings being about 98% accurate within published spec if the reading error goes in my favor. For freehand with a paint stick vs controlled lab environment, I'd consider that reasonably decent.

Light spectrometer test
(Just because I think this stuff is interesting)
The human eye can see light in the visible spectrum of 400-700 nanometers (nm). My light spectrometer scale goes just below 400 to just above 700.

Measured DeAutoLED (all the competing LEDs will be similar):
Readings in 100nm, the important part is the range under the scale on the right.


Measured HIR (all incandescents will be the same)
Readings in 100nm, the important part is the range under the scale on the right.


You can see that incandescent light sources provide a more complete spectrum of visible light than LEDs.

Why is this important?
Color Rendering Index (CRI) is a quantitative measurement of the light sources ability to reproduce colors accurately. Incandescent light sources like the stock 921 bulbs or the HIRs will have a perfect score of 100, meaning all colors are reproduced accurately. LED light sources are more commonly around 80 because they are missing parts of the light spectrum causing some colors to be washed out. While my hobbiest grade scientific light spectrometer isn't perfect, you can see the HIR has nearly 100% coverage from 400-700nm vs the LEDs with less. Look at the output pics from the bed with the HIRs to see the side by side CRI output differences.

Random LED CRI side note
I use LEDs extensively though my home for energy savings and longevity. If you go to the hardware store you can by standard LEDs or HD LEDs for your home. HD stands for High Definition, which means they usually have a CRI around 90-92, vs the standard 80 CRI. However HD LEDs are notably more expensive and rarely qualify for subsidies or rebates. So I will use HD LEDs in areas where quality spectrum lighting is important, like the dining room. And 80 CRI LEDs in areas where full spectrum lighting is not important, like the front porch or the hall way. However in the kitchen, where seeing colors 100% accurate is important, I still use halogen.


So what lights are the best?
The purpose of this thread is not to recommend any product, it is merely to be an informative thread on lighting technology with examples. Many people like the look of whiter LED light, some people like pure function over form, others will only want a true plug and play solution.

What lights are in my truck? HIR.


If you found this interesting, you might be interested in my other lighting threads:

Fog light upgrades:
The LED SAE J583 Fog Pod & Fog Light Review
The ultimate foglight upgrade H11 (not LED or HID)
The H10 to 9011 HIR Foglight upgrade (better than LED)

Other lighting upgrades:
3rd Gen HID vs LED vs Halogen H11 projector headlights
The ultimate headlight upgrade H4 (not LED or HID)
Gy6.35 HIR 921 reverse light upgrade (vs high power LEDs)

Rock Lights:
The Rock Light Showdown

More information on automotive lighting:
Automotive Lighting 101
Why LEDs should not be run in Halogen reflectors

Home lighting upgrades:
High quality efficient home lighting using LEDs, HIRs and Halogens


I'm always happy to help people on their truck projects, if you have any questions don't hesitate to PM me or reply to this thread.

 

Holy shit.... ill be reading this Monday for sure.

 

Skimming, do i not see a design with a projector lens and an array?
Ive had the projector ones and whike they throw far.... the immediate area behind and to the sides suffers.
I was wondering about a 360 array around the body of the light, with 1 or 2 rear facing with a glass projector

 

I used the TRS ones with a projector, but no additional array. As you mention, they throw far but side output is non-existent. There are a lot of designs out there, I just selected a few.

 

Yeah, i could beam the garage door across the street EASILY...
But couldnt see a thing to my immediate sides or rear.

 

Yep, as shown in the rear view cam photo and the comparison photos where the side marker is failed to be illuminated.

 

Very well written! Thanks for the write up!

 

Nice write up OP.

 

Excellent write up on reverse lights! I'm going to be reading the other lighting threads now. "I have seen the light".

 

Holly shit what a write up.

Are there any issues with the HIRs producing too much heat? I plugged a set of 100w H3 bulbs in my old Silverado fog light housing and nearly burnt the truck down. Although I suppose reverse lights will rarely be on for more than a minute

Also, any best practices for securing them in place?

 

Nice comparison/research. So best bang for the buck, Sylvania Zero 921 LED?

 

Awesome write up as usual, and great find on the HIR bulbs!

Just to add some additional data to the thread clearly from your tests the design of an LED bulb makes a huge difference as to what the light output will actually be once in the reflector. When I changed mine over a bit more than a year ago I found a design that did both side lighting for the reflector and direct lighting and also kept everything symmetric as opposed to the "blade" designs:

VLEDs 5K White Platinum 921​

These are 500 lumen and draw 7.5W per bulb.

Animated gifs showing stock only, stock+VLED and VLED only both from the tailgate and through the backup camera:

Wide angle rear view from above tailgate​

View through backup camera
​

These particular LEDs have as you can see significant light output increase over stock and a good light pattern. I think the visible output increase is down to what you illustrate in your test - arrangement of the LEDs matters a lot. Also VLEDs doesn't seem to BS on their lumen specs. If anything they are conservative and appropriately derate the output. Most people would probably claim these as 670 lumen based on the type and number of LEDs on each bulb.

These LEDs are $30 per bulb while your HIR bulbs only cost that for the pair! Given how rarely backup lights go on certainly you don't need the longevity of a LED so the HIR seem like the almost perfect option here.

 

I had accidentally linked the wrong photos (out of my hundreds of reverse photos) for the Sylvania Zevo LEDs and double posted the DeAutoLED photos, I've updated the Sylvania Zevo output photos with the correct ones.

 

See my updated pics as I initially had linked the wrong photos for Sylvania Zevo. Up to you to determine what is the best bang for the buck and what you are looking for in a backup bulb.

Very interesting find, they definitely have the highest lumen spec per LEDs and the highest wattage (which is a good sign, meaning it has the power to back up the claim if the specs are accurate). Could be some benefits of the circular design over the blade design. Maybe I'll order one to compare.

Animated gifs while flashy (good for marketing) are a poor way to show light improvement. You will never find an engineer display their work in a gif. I only say this because I have been asked. You can see in the first animated gif how the entire sky between the trees lights up as the LEDs turn on. I don't care how bright the LEDs are, they are not lighting the night atmosphere. Which then means the camera has adjusted the settings in a way that make the entire LED picture brighter, so the lights are appear artificially brighter. Which defeats the point of the entire comparison. This is what I covered under my first paragraph of light testing notes about using separate photos. I by no means say this to pick on you, I appreciate the contribution to the discussion and your LED find, I just am pointing out the lack of data a gif provides.

 

That's a standard GIF artifact. GIFs use 8b color indexing a color table as well as dithering. That hoses the shadows which is why the sky appears brighter. It however doesn't impact the mid tones which are quite accurate.

The point to the GIF is to provide a quick and accurate visualization of the change in brightness for a lay viewer because separate photos have their own issue namely the human visual system can't compare tones between widely separated regions in a field accurately and is also easily spoofed by changes in in the brightness pattern. As a result in separate images as presented a change light pattern can be mistaken for a change in brightness when viewed "side by side".

The nice thing with the separate images at higher color depth is that a knowledgeable user can open them in separate tabs and then hop between them to get an accurate visualization of the actual brightness change without being spoofed by the eyes local contrast processing. But most lay viewers aren't going to do that and will fall prey to common optical illusions of relative brightness.

Ideally a high bit depth animated image would be used - sadly the web hasn't produced a particularly common format for that.

For your images the four panels and displaying with different LEDs in each taillight is a good format. In my case the field is not uniform reflectivity on each side of the truck so I chose the animation as being easier to view.

As stated above the sky brightening is an artifact of 8b color tables used in GIF creation. The images are all taken at identical exposures and reflect the real difference in brightness. Again the shadow regions are not accurate as described. If you'd like the original full images I can upload them.

No worries! Hope I made it clear what the GIF shows and some of the pitfalls of attempting scrolling comparisons of images as many users will do.

 

Looks like I already uploaded them awhile back!







Also in these shots it should be clear how much ambient light there is around. Near to midfield illumination is clearly illustrated but farfield is pretty useless.

 

Thanks for another great write-up!!

 

Your lighting reviews would be stickies.

@tcBob

Just sayin.

 

Welcome to the challenges of light output photography.

I am familiar with subpar 8bit color indexing of gifs, which is why I point out their short coming. However, picking and choosing what parts of a test are accurate would not pass any engineering muster. The human eye certainly has difficulty comparing multiple images, especially when spread out. It is just as easily tricked by flashing images like a gif. Which is why I still believe the best low tech way to compare output is side-by-side in the same photo.

 

Now if I could just forward this to all the lifted superdutys with snowboard goggles hanging from the rearview mirror..