Axiom Home Page Forums General Discussion Home Theater Bias Light

I've mentioned a few times that I have a "bias light" behind my screen, and that everyone else should too.

I've attempted to photograph how it affects the image, but because of what the bias light is actually doing, i.e. providing a reference for the eye, the camera is not fooled as easily.

A bias light does two major things: One, it provides a reference white (provided you have neutral colored walls). Human vision tend to compensate for the color of lighting. You'll still see an egg as being white even under a purely red light. A scene in a movie that's lit by a red light will have less visual impact as your eyes will adjust. A constant 6500K reference will make the colors on screen appear more true. Two, it keeps a base level of lighting in the room. Watching a screen in complete dark will cause a constant change in the pupil where a dark scene will make a following bright scene wash out. So some room light is required, but if the light is in front of the screen it can wash out the display, and highlight other objects near the display.


This first image is taken with the over-head room light on. All the bulbs in my apartment are 5900K, 5-phosphor, CFL, with a CRI of 93. They're very good bulbs, almost as good as the bias light. I white balanced on the wall behind the screen.


The second image is with the lights out. This time I went with an auto-white balance as that's what your eyes would be doing without any other reference.


This shot is lit by the bias light located behind the TV. Again the white balance is on the wall. This light is a florescent tube, with a color temp of 6500K, and a 7-phosphor coat, with a CRI of 96. This is a bulb used to color proof commercial goods in a viewing booth. Also notice how you can no longer notice the speaker or the PS3. The rest of the room is equally dark.

What I couldn't simulate is the slow reaction of the eye's iris to changing lighting conditions in a film. If you want a better idea of the subtle differences between the shots, open them in separate tabs and switch back and forth. Notice especially with the first and last that the walls appear to be the same neutral white, but the color on the display becomes more saturated with the bias light shot.

There's also something cozy about a soft glow emanating from behind the TV.

Intersting, thanks. I may have to do something like this for the new room.

Ray3 was always a big fan of bias lights if I remember correctly....

I use an Ideal-Lume standard, 6500k bias light behind my TV. Wouldn't watch without it.

What TV is that?

Very interesting. I have never heard of this effect.

Is the type of light the bulb produces the key or the fact that it is located behind the TV the most important item or both?

Also, is anyone doing this for projection screens, or is it only TVs?

I use the Ideal-Lume Pro, but that's probably overkill. I had a Standard before, and really can't tell the difference.

The TV is a Pioneer PDP-5020FD, 50" plasma.

Placing any light behind the TV to give about a 15% gray value to the surround wall is enough to bias the eye's iris, and improve perceived contrast. The reference 6500K white with a very high color rendition index, biases the white balance of the eye to make the hues read more correctly.

Projection screens are big enough to fill the field of view that is it hard to make use of a border to bias the eye. I've also read that the larger the screen the less it is needed. But going from 32" to 50" didn't decrease the effect to me.

Wasn't there a Panasonic or Sony TV that came out not too long ago that radiated light behind the set? Only as I remember it, it wasn't a white light, it would radiate the main color on the display at the time, so it was constantly changing. In the commercial they touted the back light as making the whole experience more involving of the viewer. However it seems they missed the whole purpose of it if they were using colored lights instead of a constant white source like you are.

Very interesting indeed.

Philips offered that....

It was Phillips, and they called it Ambilight. And you're right, they missed the point. The color and intensity varied with what was on screen. So it couldn't provide a reference at all.

Very interesting, Chris. Thank you so much.

We have a 61" JVC HD-ILA RPTV. We always have some ambient light on in the back of the room; it helps a lot. Since the lights are small, halogen pucks firing at the white, cathedral ceiling, there is no glare on the TV.

I'm curious about how the color of the wall behind the TV might affect this concept. Painting the interior of our house is slowly working its way up the list, and I had been thinking of going "dark" behind the TV, but your post is making me rethink that.

SO.....if we just went standard, are we talking 25w, 40w, 60w....?

1.21 Gigawatts.

You don't want an incandescent bulb for this. Their color temp is way too low, it'll bias your eyes to make everything appear blue on the screen. You need a "daylight" bulb, the closer to 6500K (which is what the color temp of your display should be set to also) the better.

As for lumens, it varies based on the distance to the wall. If you have any DVD/BD with test patterns look at the 15% gray, that's how bright the wall should appear behind the TV. That is one nice thing about the Ideal-Lume Pro, it has a dimmer so I could dial it in exactly. Is that worth the 500% increase in price over the standard? No.

Excuse my ignorance - so would one of the Fluorescent bulbs - low wattage (for lack of a better term) in one of the configs like "Daylight" aiming at 6500K is what I am getting out of this?

There's two things which determine the quality of a light source that's claiming to be "daylight". First the color temp, daylight falls in the 6500K range, and that's exactly the number you want for a video display. The other is CRI (Color Rendition Index), the sun gets a CRI of 100, because that's the reference. It's basically a measure of how complete the spectrum produced by the bulb is. Good bulbs are just above 90, excellent in the mid-90s, and crazy expensive in the high-90s.

If you can find a low-lumen, fluorescent bulb with a color temp of 6500K, and a CRI over 90, you'd be in good shape. If you just want to get an idea of what the light would bring to your display, go with any daylight CFL. But at $60 for a 10,000 hour bulb, plus fixture, the Ideal-Lume Standard is a great deal.

Thanks. I knew I would learn something today - did not know about the CRI..........Rob

The problem with using an excited phosphorescent material for a light source is that it isn't black-body radiation, but rather conforms to a more coherent spectrum. So florescent lights trying to mimic white light will use several different materials in the coating, each with a different spectral center. The overlapping spectra produce something closer to "white" light.

Cheap bulbs will use 2 or 3 phosphors and produce a very poor, tinted light (sub 90 CRI). At 5 different phosphors florescent bulbs take on a different character and become very high quality light sources (94+ CRI). At 7 (96 CRI), they best everything but the $1500 filtered tungsten halogen-cycle bulbs.

They don't call you ClubNeon for nuthin', do they?

I've got a 24" Florescent light that is rated at 4200K... is that too cool?

In the '70s it was called a disco ball...

You can go to either the GE or Phillips website and search through their offerings to find an appropriate bulb. There are a number of 'aquarium bulbs available that are 65K and 90+ CRI.

David, 4200K is warmer than the theoretically desirable 6500K output. Higher numbers are cooler.

Thanks for the info about cooler/warmer... I never knew that.

Anyway, I went ahead and sprung for an ideal lumen standard for 60 bucks from cinema quest. Probably could find something cheaper but I figured I'd go with something that someone around here has used with success. Thanks for all the info. I look forward to setting things up!

Thanks for the informative thread, Chris. I may look into getting a light at some point, even if my TV ends up in the bedroom after I get a projector.

I wonder if antique white counts as a "neutral" wall color...?

Actually, color temp, is in Kelvin, so lower is cooler. But as the color temp increases it turns more blue from red. Artistically blue is considered cool color while red is warm. Just think the hottest part of a flame is blue.

It's probably better to just say, "4200K is more reddish." Than use "cool" or "warm" as they have opposing meanings to different people.

Yeah Chris, but since the visual effect rather than the temperature as such is the crux of the matter, the convention of referring to the higher color temperature as "cooler" and the lower color temperature as "warmer" is in common use and the one that I follow.

Oh boy the gloves are coming off now! This is almost as good as Hockey!!!

Yes, when discussing color temperature, cooler means "more bluish" and warmer means "more reddish".

It is true, however, that just like the hottest part of a flame is where it is bluest, the cooler looking colors correspond to wavelengths that are generated by hotter black-body radiators. (see above link)

I think red cars look really cool.

The color professionals I talk with never use "cooler" or "warmer", because it is just too confusing. They'll say "lower" or "higher" color temp, or redder or bluer shift.

Every occurrence of "warm" in that Wikipedia article is in quotes, and immediately followed by "lower color temp".

When I want to be warmer, I find a woman. When I want to be cooler, I marry that same woman.

Wait, I just realized sexist comedy isn't funny, and it would have been more relevant, anyway, if I was basing my observations on the woman's color.

Racist comedy isn't that funny either.

Out of the frying pan and into the fire is how I do things.

More temperature references?

When you're hot, you're hot.

That's not another chicken joke - is it?

Burn in hell.

Where are your manners? You should have said burn in hell, please.

Bun in hewul, bleeze. </E1>

Thanks to this forum, I spent some more money! Just ordered the Ideal-Lume Panelight for my LCD upstairs. I'll see if it makes a big difference or not and report back.

Yeah, when I get my Ideal lumen standard I'll report back with my findings.

You got something against chickens?

Anyone try this with some high CRI/high temp LED's? I'm guessing more efficient with easier dimming via high cycle PWM.

Are there high CRI LEDs? While phosphorescence is nearly coherent, the light from an LED is truly. So I imagine to get any where near a 90 CRI you'd have to cluster more than 3 colors of diodes, just as one has to do with the florescent coat.

Cree and Seoul make some high CRI LED's, but I'm not sure if they are also high temp. Apparently the phosphor technology has improved a whole bunch, because AFAIK it is still the same yellow phosphor over blue die. I know some of the newer stuff can put out 120+ lumens with a little over 3V and 400mA or so.

Here at the Axiom forum, one of our favorite things is spending someone else's money.

Oh, I wasn't thinking about that LED technology. They are UV LEDs which then excite a phosphor coat. So they are basically florescent bulbs, but with a diode junction instead of a discharge arc creating the ultraviolet light. With a good quality coat, they could indeed be high-CRI.

Here's an article that showed up recently where someone put lighting around a 110" screen: http://www.electronichouse.com/article/theater_images_pop_on_floating_screen/

It wasn't designed specifically as a bias light, but for looks. It does look nice. If the area around the screen was painted a neutral white, and quality bulbs were installed, I think that would work well.

That theater looses points for having a Monster product in the rack, but he breaks even for that screen.

You know this thread has made me realize that I sort of made the same discovery as you, only accidentally, and not exactly in the same way. You see when putting together this room we put track lighting all around the room. Well when it's movie time we turn the center 'pot lights's completely off, but I prefer to keep the track lighting on just slightly. The walls are an off-white color, and I don't know the temperature of the track lights, but they create an effect similar to the environment you discribe.

Because of the way my screen is forward from the main wall about 4 feet, and the track lighting is directed at the walls on either side of the screen, it basically creates this effect in a round-about sort of way. Like I said it's not exactly the same, but similar. I've tried turning all of the lights off during movies, but for the most part I do prefer having the track lighting on low. I've never noticed that it might create better/deeper color saturation appearance or any of that other stuff, I'll have to pay closer attention next time I watch a movie. Maybe I'll turn them on and off and see if the screen characteristics change.

Interesting stuff.

Got my ideal lume standard from cinema quest today. Got her hooked up on the back of my TV and adjusted the brightness to just below the reference ambient light pattern on the DVE calibration DVD.

So far it seems pretty nice. If nothing else, it provided a comfortable amount of light in the room with no glare.

You're closer to CO! Mine gets here tomorrow.

Yeah, I am in Oklahoma. Did you order on Friday as well?

I plugged my light into one of the secondary plugs onto my APC unit that is controlled by the master plug (where my receiver is plugged into). Now, the light automatically comes on when I turn on my receiver so that is pretty cool. I had to dim down the light quite a bit with the built in "dimmer" (its just a dark piece of plastic or something that you can rotate around the light bulb to adjust the brightness.) The dimming feature is very basic but works surprisingly well.

I am glad that I made the purchase.

Yup, same as you. Even if it came today, wouldn't been able to hook them as I'm watching the Yankees cruise to their 27th championship! It hurt, but I sold my game two tix. I was at the stadium form ALCS clinching game, so it was good enough for me.