Uneven light on screen

[Richie May]

With a white field up, the light is brighter on the left side of the screen in flat as well as scope by 4 foot lamberts. Top and bottom are even. How do you adjust left to right brightness? Projector is a Christie CP4220. LOC calibration is done.

[Marco Giustini]

what screen surface are you using? Anything but a matt white screen (gain = 1.0) will give you uneven light - though it shouldn't be left/right but it will change depending on where you are looking from.

There is really no adjustment for left/right brightness in the projector besides the lamp adjustment. IN my experience the automatic process is usually quite accurate with Christie but you can always tweak it manually while looking at the screen.

[Ioannis Syrogiannis]

Marco is right. That is probably the reason.
Factors like the projector being off-center could also explain such discrepancies. Mostly when the lens is shifted, to compensate.

The port glass, or the lens' condition could have something to do. Do you maybe see any difference when you put up test patterns other than white? (Not by measuring, with naked eye.)

[Richie May]

It's a Matte white screen. Projector is dead center. No keystone. Optical glass in port. I've tried the manual X,Y,Z controls but they don't do what I would expect. I assume X and Y are up, down, left, right. Z is focus. They don't seem to work that way when I adjust them.

[Marco Giustini]

Well, a digital projector is not affected by lamp alignment like a 35mm one. Light in a Digital one goes into the "integrator rod" which basically mixes it up. So if the lamp is misaligned you mostly end up with less light but still even-ish. Depending on the projector you should be able to see some shadows though, i hoped that could have been enough in your case.

Ioannis has a point: lens shift. Yes you said the projector is dead centre but... can you run a white field and take a photo of the light coming out of the lens from the auditorium possibly? I'd like to see if the light is also centered.

Failing that, it could be a bad mirror?

One quick test to rule out things is to move the white pattern left/right/up/down: does the unevenness move with it or stays there? That would rule out the screen.

[Steve Guttag]

On a Christie, this would point to a reflector to light pipe misalignment (for Barco or NEC, it would point to a cold mirror misalignment) or reflector damage.

[Ioannis Syrogiannis]

That is because (contrary to how it was on film) the light path includes mirrors and the integration rod, that translate the movement to be on a different axis and the light to be more homogenous towards the image panel. You don't have (that much of) those dark corners that existed back in the film days due to bad lamp focus.

Matte white could mean a gain other than 1, as a worn out or dusty screen could mean loss in brightness.
Lens shift you may check as well. No keystone may be the result of shifting the lens instead of turning or tilting the projector.
Optical glass (port) may have accumulated dust or other dirt. Same with lens. There might be obstacles.
I remember seeing a dark corner somewhere, and it turned out to be a cable obstructing the scope beam that would look as a smudge on the screen.

When you cover the external factors, then go inside. Where checking the primary colors' test patterns would make sense.
Same with checking the light path from lamp, lamp mirror, cold mirror, integration rod, fold mirror, light engine, lens.

My personal opinion is that chances are bigger to find your explanation within the external factors.

(I see that others beat me to it. Nothing new.)

[Ryan Gallagher]

In addition to what others said.... Method of measurement is also in question. What meter/device are you using? Are you seeing a difference or just measuring one?

If you are a tech you probably have all this covered, but if you are an operator or projectionist, worth asking. Are you measuring from a location that is equidistant from the two screen points (assuming some kind of spot measurement meter). Is this a reflected light measurement or are you doing some math with incident light meter, or god forbid some kind of phone based light meter?

[Marcel Birgelen]

Like Marco indicated, the integrator rod helps pretty well to even out any inconsistencies on the lamp side. But I've had a similar problem with a very visible diagonal falloff of brightness on a Barco 3DLP big venue projector, not that long ago. In that case, it was the integrator rod itself that needed replacement.

[Hong Sung Hyeok]

Brightness uniformity is significantly affected by how much lens shift is used. Peripheral brightness degradation due to aging is out of the question.

If without keystone, wouldn't the lens shift be biased?

Check to see if the image is focused on the center of the lens. The center of the lens is bound to be the brightest.

While the positioning of the screen and projector is important, it can also be a matter of preference for the projector installer: whether to lose brightness and eliminate keystone, or gain brightness and mask the lower left and right corners with keystone.

Personally, if it were a new laser projector, I'd set it to be keystone-free. However, if it were an older lamp projector, I'd reposition the projector to secure brightness.

[Marco Giustini]

I've never noticed the brightness being so affected by lens shift, AS LONG as you stay within the recommended range. I've seen plenty of installations where the light beam was being clipped by the lens barrel itself, hence my previous question.

What I'd normally do is to ascertain the maximum amount of lens shift the system can give without noticeable compromises and then complete the alignment by mechanical adjustment and crop.

[Dave Macaulay]

I have had 2 systems with light pipe damage, there are lenses as well as the integrator rod in a Barco at least. Both times a lens was cracked. Light pattern was far from flat.

[Hong Sung Hyeok]

The idea that the left side is brighter also means that the center of the light source is on the left. I suspect the lens shift is too far to the right.

If the integrator rod is broken, it should indicate something other than just a dim or bright screen. You might see dark spots or cracks along the edges of the screen.
Or try manually adjusting the lamp X & Y axis to see if the brightness uniformity changes.(without Z)

[Marco Giustini]

I had a Barco cracked lens inside the integrator as well - I don't remember seeing anything on screen, I looked inside to investigate a separate issue which was not resolved with a new lens. I don't even remember seeing an improvement in light output!