Tweet
Recently, my one degree spot meter's electronics failed with no possibility of repair. This left two incident light meters available, each capable of reading luminance in lux. One is a very capable Minolta chroma meter (extremely expensive). The other a simple lux meter which can be purchased new for around £50 here in the UK. The Minolta does indeed have the ability to measure luminance as one of its functions so I attempted a procedure to give an approximate measure of screen luminance from the center seating position as follows.
From approximately the center of the screen and very close to its surface, obtain the maximum reading possible for the reflected screen light using a 100% white projector test pattern. It is necessary to take a number of readings and try various distances from the screen to obtain this maximum, minimizing effect of the meters own shadow. This turned out to be about one foot from the screen surface. The reading would be expected to be slightly low due to the shadow of the meter itself. This reading (in lux) can then easily be converted to foot lamberts by dividing by the constant 3.426, in the UK we also tend to measure screen luminance in FL rather than the S.I. unit candela per square meter. Our normal target is 14fl.
Now the meter must be converted into a pseudo spot meter, at least as far as the central area of the screen is concerned. A cardboard tube (a toilet tissue center core was perfect) is fitted over the collection hemisphere to restrict its angle of acceptance. A new reading is obtained from the position close to the screen used for the measurement in the first step. The value has no absolute meaning but will be used to calculate the drop off ratio. Now measure again the maximum brightness from the central seating position, the screen being big enough to provide 100% coverage of the meters restricted view. The ratio of the two is used to reduce the FL figure obtained in the first step to that for the center seat.
This method does seem to give a very good approximate reading for screen luminance. Interestingly the very cheap meter (stated accuracy +/-3%) was very close indeed to that of the Minolta.
The Minolta did also detect a very slight mired shift in the projector optics or perhaps screen surface which was easily corrected via a software adjustment and the colour temperature of two projectors found to be within 50 degrees kelvin of each other taking into account what ever pollution was present on the screen surface.
Chris Woollard Projected Picture Trust, UK
From approximately the center of the screen and very close to its surface, obtain the maximum reading possible for the reflected screen light using a 100% white projector test pattern. It is necessary to take a number of readings and try various distances from the screen to obtain this maximum, minimizing effect of the meters own shadow. This turned out to be about one foot from the screen surface. The reading would be expected to be slightly low due to the shadow of the meter itself. This reading (in lux) can then easily be converted to foot lamberts by dividing by the constant 3.426, in the UK we also tend to measure screen luminance in FL rather than the S.I. unit candela per square meter. Our normal target is 14fl.
Now the meter must be converted into a pseudo spot meter, at least as far as the central area of the screen is concerned. A cardboard tube (a toilet tissue center core was perfect) is fitted over the collection hemisphere to restrict its angle of acceptance. A new reading is obtained from the position close to the screen used for the measurement in the first step. The value has no absolute meaning but will be used to calculate the drop off ratio. Now measure again the maximum brightness from the central seating position, the screen being big enough to provide 100% coverage of the meters restricted view. The ratio of the two is used to reduce the FL figure obtained in the first step to that for the center seat.
This method does seem to give a very good approximate reading for screen luminance. Interestingly the very cheap meter (stated accuracy +/-3%) was very close indeed to that of the Minolta.
The Minolta did also detect a very slight mired shift in the projector optics or perhaps screen surface which was easily corrected via a software adjustment and the colour temperature of two projectors found to be within 50 degrees kelvin of each other taking into account what ever pollution was present on the screen surface.
Chris Woollard Projected Picture Trust, UK
Comment