Ad-Hoc method of calculating lumens - AVS Forum | Home Theater Discussions And Reviews
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Ad-Hoc method of calculating lumens


Users trying to calculate lumens values without a luxmeter or another device to measure lumens can use these methods:
Does not have all models, and differs slightly from manual calculations.

If a luxmeter or something similar is available, measure lumens and divide that by the square surface of the screen. The measurement can be taken from the light reflecting from the screen.
To account for variance in brightness uniformity, a checkered black and white pattern can be used. Take a measurement in each white rectangle, and calculate for the average value.

The general recommended fL value is 12-15fL for SDR, and 30+fL for HDR.

The environment matters, the image will be perceived brighter in a treated room (wall reflections, light control), than a room with colored walls and other light sources.

Manual method:

1.Take the dimensions of the screen:

2.Find out the square surface:

Note: a lot of projectors output an image in 16:9 format. This can be changed with an anamorphic lens.
If the projector used outputs a 16:9 format image, and the screen is, for example a 2.35:1 format, the square surface used for calculations is for the 16:9 screen in which the 2.35:1 image fits into, not the 2.35:1 screen. The black top and bottom bars still are projected, just outside the screen, and the lumens value includes these two areas outside whatever screen configuration is used.

3.Find a review with measured lumens.
Projector Central reviews usually has measured lumens value. The lumens value is in the position with the most zoom.

4.Going from most to least zoom has a negative effect on lumens.
Projector Central reviews also measures how much of the lumens percentage is lost.

5.Calculate how much lens zoom is used.
Use the Projector Central Calculator:

6.Calculate lumens loss due to lamp use.

UHP lamps loose ~25% if their brightness in the first ~500h. After that it's a linear curve to the estimated life of the lamp, which is 50% of it's original brightness.
It's not clear if the 25% loss after the 500h is dependent on the lamp mode used.
Calculations can be done with removing the 500h from each rating.

Full lamp rated for 3500h, Eco for 5000h.
If using the Full lamp rating, remove 500 from 3500. After this period the lamp has lost 25% of it's brightness, so for the remaining 3000h of the lamp's brightness will decrease linearly from 75% to 50%.

Different lamp modes affect the lamp's hour timer depending on the brightness.
If the lamp was used in different modes, take this information from the projector menu and calculate an equivalent hour value for whatever lamp mode/rating is used (e.g. 3500h for Full lamp) . This menu should detail how many hours were used in each lamp mode.

For instance, if the lamp's brightness on Eco is 70% of the Normal lamp, calculate the hours equivalent to Normal, or vice versa, depending on which hour rating is used.

After the initial 500h, if the lamp has been used for an aditional 400h in Full and 1000h in Eco.
Equivalent hour rating would be:
For Full lamp mode rating of 3500h (3000 without the initial 500): 700h (1000h in Eco is 700h in Full) + 400h = 1300h
1300h is 43% out of the brightness after the 500h (75%) and until 3000h(50%), or 10.75% out of the lamp's entire (0-100%) brightness. So after 500h (initial) and 1300h (Full lamp equivalent) the lamp would be at 64.25% of initial brightness.

7.Calculate fL for desired mode (Cinema, Bright, etc), and lamp mode (Normal, Eco) by dividing the square surface to the lumens value.

8.Calculate to nits if desired:

9.Adjust for screen gain.
Manufacturer gain is not always accurate, so try to find a source that has measured the screen, like a review or a forum thread.
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