Film-Tech Forum ARCHIVE: High Contrast Light Loss

This is topic High Contrast Light Loss in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Muhamad Taufiq (Member # 8868) on 08-14-2018, 12:16 AM:

I'm a little bit curious, since after searching around here and Google I can't seem to find definitive answer about this topic.

When comparing between High Brightness and High Contrast lens, how many fL (or lumens %) can you expect to lose?

And then as I understand some (or all) manufacturers provide iris to increase contrast, how many light loss to expect?

Posted by Steve Guttag (Member # 268) on 08-14-2018, 07:13 AM:

Unfortunately, I don't think there is a set figure. Christie has, traditionally, offered High Contrast lenses for their 1.2 and 1.38" DLP projectors and how much light you lose is probably dependent on the focal length. It would not surprise me if it was on the order of 20% or more.

Barco only offers the High Contrast lenses on their "P" (post-production) and Laser projectors with the claim that the heat from the other projectors would damage them. I know one gives up a lot of light on the "P" version of their projector but that is for the whole light path, not just the lens.

As for an iris, Christie has offered them for their 1.2/1.38" machines and it is a kit of several sizes so you can choose what light-loss versus contrast improvement so again, it is variable.

NEC, on their .98 (and maybe larger) comes with a single iris that seems to knock off about 20% of your light (I didn't really measure the one I used...I had too much light on a small screen so I used it to get the light into the range). That theatre is no-more so I couldn't measure it now.

Posted by Tom Bert (Member # 5719) on 08-14-2018, 09:21 AM:

Barco has 3 types of lenses: High Brightness (as in our standard projector models), High Contrast (as in DP4K-P) and Very High Contrast (as in DP4K-LHC and BLPHC models)

On a standard projector you can switch between the standard HB and HC lens. Going to the HC lens will boost contrast by ~25% and drop brightness by 20-30%.

Going to a setup with VHC lens always happens together with adaptations to the internal light pipe. It yields a contrast boost of more x2.

Posted by Muhamad Taufiq (Member # 8868) on 08-14-2018, 10:47 AM:

Thank you Steve and Tom.

Posted by Randy Stankey (Member # 64) on 08-14-2018, 06:23 PM:

If I was doing this with traditional camera lenses, I would compare the f-numbers and focal lengths of the lenses and calculate the difference in the light.

You would also need to factor in the light transmission qualities of the lenses in question. (e.g. Better quality glass, coatings, the number/position of lens elements and overall design.)

All other things equal, you can simply divide the focal length of the lens by the diameter of the lens opening to get the f-number (if it isn't already specified) and compare the two. The lower f-number would give better light.

This is a very simplified overview and doesn't take a lot of factors into account. (Such as transmission qualities.) However, it will give you a rough idea of the differences between two lenses.