Slides:

2 slide projectors used 250W bulbs, projecting a 6-7’ width picture, approx 2.5sqm area. Both read EV7 brightness, and were considered acceptable.

3 slide projectors used 250W bulb, projecting 120cm wide, approx 1sqm area, and readings 9.2-9.8EV. The higher level was considered usable but too bright.

1 slide projector used 150W bulb, projecting a 9.5’ width picture, approx 5.5sqm area. This read EV6 and was consider slightly dim.

Early thoughts are that measuring brightness with a blank slide mount in the gate does not allow for light losses for even a clear piece of film. However, one respondent has checked, and found no measurable difference.

Digital:

One projector of unknown lumen rating (HPvp6121) projected a 200cm wide, 3sqm image, with EV6.5 brightness, and this was considered acceptable. This was after setting the brightness and contrast using the RPS grey step wedge.

One projector of 2000 lumen rating projected a 275cm wide, 5.67sqm image, at a Federation event, with EV6.7 brightness, and this was also considered acceptable.

One projector, rated 1800 lumens, projected a 114cm wide, 1sqm image for a smaller Club, with EV9.5 brightness. This was considered a little bright. By calculation, if this projector was used to project a 5.67sqm image (as above), then it might have EV7, suggesting that an 1800 lumen rating is still adequate for Federation use.

One projector, rated 3500 lumen was considered too bright at full power on a 180cm wide, 2.4sqm, screen, but satisfactory in quiet/economy mode. Readings on the 180cm wide screen were EV7.5, and on a domestic 90cm wide, 0.6sqm, screen EV9.5. The difference of 2EV is as predicted by the x4 relative screen areas.

Monitor:

A CRT monitor, used in general room lighting, and considered of adequate white level, read EV9.5. In a blackout, this might be too bright, which would agree with other projector readings and opinions.

Comments:

With limited data to hand, an acceptable brightness for digital projection may be in the region of EV6.5-8.0. EV7 may be a minimum for slides.

1800 lumens may be adequate for Club and Federation use. Digital projectors can be underrun, so that a higher lumen specification should also be satisfactory if economy mode is used. No upper lumen limit has been identified yet.

Higher EV (up to 9.5) have been found acceptable for projection on smaller screens eg, 4’ wide, approx 1sqm. It is possible that a higher white level is acceptable precisely because there is less screen area, and hence the observer’s total experience is a smaller area of high brightness in the typical blackout. If so, then this would be a favourable situation: using the same projector for a larger group, entailing moving the projector back to get a larger screen image, would still appear acceptable to the audience even though the image is objectively dimmer.

But, we also know that certain Clubs at national slide events bring individual images at a range of densities. Their experience suggests that there is an optimum brightness for any given screen image size. While authors of projected digital images have been concentrating on colour rendition, there may be a separate issue of image density which would depend on how large the event organisers choose to project the images.