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Bias Light
#276778 10/30/09 06:13 AM
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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.


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-Chris
Re: Bias Light
ClubNeon #276779 10/30/09 06:20 AM
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Intersting, thanks. \:\) I may have to do something like this for the new room.


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Re: Bias Light
jakewash #276793 10/30/09 10:33 AM
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Ray3 was always a big fan of bias lights if I remember correctly....


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Re: Bias Light
MarkSJohnson #276795 10/30/09 12:14 PM
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I use an Ideal-Lume standard, 6500k bias light behind my TV. Wouldn't watch without it.


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Re: Bias Light
Ajax #276796 10/30/09 12:25 PM
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What TV is that?


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Re: Bias Light
Ajax #276797 10/30/09 12:25 PM
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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?


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Re: Bias Light
Murph #276805 10/30/09 12:54 PM
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Also, is anyone doing this for projection screens, or is it only TVs?

Re: Bias Light
Murph #276806 10/30/09 12:55 PM
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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.


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Re: Bias Light
CV #276807 10/30/09 12:57 PM
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 Originally Posted By: CV
Also, is anyone doing this for projection screens, or is it only TVs?


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.


Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris
Re: Bias Light
jakewash #276810 10/30/09 01:33 PM
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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.


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