Lux / Lummen Difference and measurement??

C

compusolve

Guest
Can any one tell me what a projector giving out 800 Lux is relevant to Lummens? I have read about the nine dot thing but what does this mean and is 800 Lux reasonably high,

P.S Gordon Stereo/Stereo

Thanks for the tip re screen, I have purchased a 2m electric from Ellie suppliers, works a treat and looks good,

Regards

Garry
 

Oasis

Standard Member
A projector's output should actually be measured in Lumens not Lux.

800 Lumens would be middle/high for CRT but pretty low for LCD or DLP.

The relationship between Lux and Lumens is that the Lumens measurment is the amount of light put out, whereas Lux is the amount of that light falling on a certain area. So a 1000 Lumen projector projecting on a 10 square meter screen will be illuminated to 1000/10 = 100 Lux.

To put this in context, Cinema projection specifications talk about screen illumination in terms of Foot/Lamberts, (Basically the Imperial version of the Metric Lumens)The usual spec being 14fL. To convert (roughly) Lux to Foot Lamberts, simply divide by 10, so in our example above 100 Lux would give approx 10fL - which is the bottom end of the cinema specification.

Light meters read the light in Lux because they have a known size of sensor area, which can be related to the light amount per square meter. So to measure the light output of a projector, it's set up pointing at a screen (You should use a black screen to avoid reflections). The projector should be set up to give the maximum usable white level, using an external source (Maximum possible and maximum usable white is not always the same thing - particularly on LCD). The area of the projected image is then measured to give an area in square meters. 9 measurements are then taken (top left, centre top, top right, centre left, centre, centre right etc..)and are added together and divided by 9 to give the average Lux reading. multiply this answer by the square meter measurment to get the ANSI Lumens.
This is the 'correct' measurement technique for LCD, DLP and D-ILA projectors.
CRT projectors are different in that they cannot provide a complete white screen at the same brightness as if it was a small area of white, so they measure the brightness by using a white square, occupying 10% of the screen area, and a single measurement is taken from that, and multiplied by the total screen area.

Where ANSI is not mentioned with the Lumen rating, you can never be sure how it was measured - the usual trick is to use only the brightest measured point on the screen instead of the average.

You should also be aware that for most projectors the quoted brightness is often the best acheived with a perfect lamp in perfect conditions. When I used to be involved with the Infocomm show in the US, if a projector manufacturer wanted to quote the brightness of a projector in the shootout (loads of projectors side by side, identical sources, identical screens, in near total black out!)they had to be measured by the ANSI method, only one projector manufacturer stood by their specs and had their ratings quoted.
One year, the organisers did independent measurements of all the projectors on show,a couple of times each day, and then published the results - it was interesting to see how far some measurments were from the specification sheets! (some were less than half the specified - and this was in a show where you expect to bring the best you have!!!)

I hope this is of use,

Oasis
 
C

compusolve

Guest
Thanks Oasis

Thats just the info I needed to Know
Regards
Garry

[ 21-05-2001: Message edited by: Garry Smith ]
 

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