The EIA Video Systems Committee set five measurement parameters that would be evaluated under low light conditions.
1. Luminance level
2. Black level
3. Luminance signal-to-noise ratio
4. Chroma level
Tests were developed to measure these parameters and minimum limits were set on the measurement results. To be judged acceptable, the camcorder would have to meet the minimum limits on all five parameters.
The Actual Tests
Each camera is aimed at a test chart with its zoom lens adjusted between wide angle and half-way zoomed in. This achieves respectable light transmission. The chart is illuminated with 3100 degree Kelvin (3100Â° K) video lights adjusted to spread the light evenly over the chart. A digital light meter is used to accurately determine the illumination of the test chart. The camcorder circuitry is exposed, if necessary, so that the video signal can be measured directly using a video waveform monitor, video noise meter, and high resolution video monitor.
The camcorder is switched to its full AUTOMATIC mode for exposure and color balance, and its GAIN UP control is set to the “normal” position. (The manufacturer is permitted to perform these tests in the GAIN UP position as long as it indicates in its specifications that this position was used during the test). The camera is manually focused and set to 1/60 second shutter speed. The electronic image stabilization circuits are switched off, the digital zoom is switched off, the on-screen display (OSD) feature is switched off, the RF adapter (TV channel 3/4) bypassed, and of course, any built-in accessory lights are deactivated.
Test 1 Luminance Level
The camera is aimed at a logarithmic gray scale chart (a chart with two horizontal rows of eleven progressively darker gray bars and a white bar in the center) and illuminated to 1000 lux, as measured with a digital light meter. Under normal light, the white chip will appear white on a TV monitor and will trace a plateau at 100 IRE on the waveform monitor scale, representing pure white. The progression of darker bars appear as
stairsteps on the waveform monitor, with the bottom step at about 7 IRE. The light level on the chart is then reduced. As this happens, the TV monitor picture gets darker and the trace on the waveform monitor gets lower. Eventually, the trace that represented the white bar only reaches 50 IRE, the cutoff point. Put another way, when white things look 50% white (50 IRE), experts consider the picture to have minimum acceptable brightness. The technician, fumbling in the dark, locates his light meter and takes another lux reading.
Test 2 Black Level
The darkest bar on the chart is 2% white and appears as a black bar on the TV monitor. On the waveform monitor the signal makes a trace at approximately 7 IRE on the scale. Cameras measuring below 4 or above 10 on the scale for black level do not qualify for further testing (these cameras, since they are not making correct black, cannot have whites that are trustworthy).
Test 3 Luminance Signal-to-Noise
Next the gray scale chart is removed and an 18% gray card is put in its place. The camera now sees nothing but dark gray. The video noise meter then measures the video signal and a calculation is made which determines the signal-to-noise ratio. If the S/N ratio is 17 dB or greater, the picture is considered acceptable. Incidentally, the lower the S/N ratio, the grainier your picture, and a number below 17 represents a picture that is quite snowy. Remember earlier how professional TV cameras sported S/N ratios around 63 dB? That’s one reason why professional video looks so good compared to what the rest of us make at home.
Test 4 Chroma Level
The gray chart is then removed and replaced with a Macbeth color checker chart, a chart with little colored squares. The chart is illuminated to 1000 lux and the chroma signal (the color part of the video signal, the C in Y/C signal) is sent to the waveform monitor. The waveform monitor is adjusted so that it measures just one square, the pure red square on the chart. The technician notes the signal level of the red square and then decreases the illumination down to where it was set earlier in test number one. With less light, the chroma signal becomes weaker on the waveform monitor. The minimal acceptable chroma level for the red square with low light is 25% of the chroma level when the red square was fully illuminated.
Test 5 Resolution
A test pattern with converging thin lines replaces the previous chart and the illumination is cranked back up to 1000 lux. Where the lines are far apart, they are easy to see on a TV monitor. Where they are close together, they merge into gray mush. At some point the technician can tell where the lines are so close together they just begin to comingle. Numbers on the chart tell what resolution that part of the chart represents, so the technician just reads the numbers off the chart. Next the illumination is decreased to the low light level determined in step number one
and the technician again checks for where the lines blend together and reads the new resolution numbers off the chart. These new numbers must be 70% of the value he/she read at full brightness.
If the camcorder passes all of the above tests, then the light meter readings in step 1 can be used as the minimum lux ratings for the camera. If the camcorder does not meet all of the minimum performance levels in tests 3, 4, and 5, then the illumination level must be increased in step 1 and the whole process repeated. Eventually it is possible to apply enough light so that the camcorder passes the minimum requirements of each
test, and that chart illumination level is the one used to describe the camera. This may seem like a lot of technobabble, but now you know how honest light sensitivity measurements are made. Thanks to the EIA and the participating manufacturers, buying a camcorder just got easier for those consumers who know to look in the specs for low light sensitivity (measured by the EIA Standard).
Dr. Peter Utz