|
PROJECTOR LIGHT OUTPUT Kodak Ektagraphic vs. Digital Projectors
Prof. John Hart Program in Atmospheric and Oceanic Sciences University of Colorado Boulder, CO 80302 December 2001 nimbus.colorado.edu/hart/science.htm
The AV committee in our Program compared the light output of a modern slide projector with two digital projectors. The goal was to see how much brighter digital projectors are, as a guide to purchasing equipment for classroom use. The equipment tested was: 1) Kodak Ektagraphic AMT III with EXR built-in "high bright" module. f 2.4 lens. 2) Sanyo XP-10NA XGA digital projector (1900 ANSI Lumen, 350:1 contrast ratio). 3) Epson Powerlite 7700p XGA digital projector (3000 ANSI Lumen, 400:1 contrast ratio). Tests were made simply by measuring the center-screen illumination (for equal width images) with an incident light meter. Results were recorded as EV at ASA100 (which can be converted to Lux). But here, we are simply interested in the relative intensity of the three units. An empty slide frame was placed in the Ektagraphic, while a pure white (RGB 255's) was sent to the digital projectors.
* With polarizer (Hoya, linear) over the lens. Polarizer adjusted to give pure white light. Conclusions: Although the digital projectors seem to look brighter (probably because the color temperature is 6000K vs 3500K) the Sanyo and the Ektagraphic are essentially the same. Thus the Ektagraphic outputs about 1800 ANSI Lumens. This compares favorably with the Lux estimates published by Navitar, and with Kodaks own estimates of 1300 lumens (with f3.5 lens, higher speed lenses were used here). The Epson is brighter by approximately 0.8 stops. Note that the Epson/Sanyo ratio is 2^8.0/2^7.3 which is almost exactly 3000/1900, the published ratio of digital projector intensities ! Placing a polarizer in the beam (as needed for 3D projection ,for example) reduces the Ektagraphic's intensity by 1.5 stops. This is the standard "compensation" for a polarizer. However, the polarizer in front of the LCD's causes 1.8 stops of light loss. I am not sure where the extra 0.3 EV loss comes from. The LCD projectors have their green beams vertical and their red-blue beams horizontal in the output. The polarizer axis must be placed at + or - 45 degrees to obtain a pure white output. This is good because standard viewing glasses have this type of polarization. For polarized digital projection of 3D using this method, you will need a projector of about 2200 to 2300 Lumens to equal the Ektagraphic (which, again, has a redder beam). A 3700 ANSI Lumen projector, polarized as above, would be noticeably brighter (by almost 1 stop). We have not yet tested to see if polarizing a DLP or DILA projector results in 1.8 or 1.5 (or other) stops of loss. Theoretically a DLP should suffer only 1.5 stops, but DLP projectors are not as efficient and so tend to output fewer Lumens per $ (as of Dec 2002). DILA is an unknown at this point. NOTE: 3D projection with the above two LCD projectors appeared surprisingly good, after the images are tweaked for color, registered, and balanced in software. Even at XGA the screen takes on a glass-like appearance where the barn door LCD pixels tend to disappear. |
