The following knots need to pay attention to in order to get the accurate photometric data:
a) Luminosity measured distance
The luminous intensity measurement is to measure the illumination of a certain distance; the light intensity value can be got through calculating the inverse distance square principle in photometry, namely I, E, R2. In this formula, I is the light intensity in the testing direction, E is the illumination of the photo detector receiving surface. R is the testing distance. However, for many lamps, especially the LED lamps, the near field luminosity law is not applicable. CIE files have made clear regulation that the luminous test distance of the lamps should be big enough (CIE Technical Report, The Measurement of Absolute Luminous Intensity Distributions, and [CIE Pub.NO.70]), and meet the following conditions:
For fluorescent lighting fixtures：R>D×10
To cast light lighting fixtures：R>D×200/θ
b) Photometric measurement accuracy
Photometric detector is an important part of the photometric distribution, the spectral response S (λ) precision of the detector should be in accordance with the human eye visual spectrum light efficiency function V (λ), namely S (λ) = V (λ). According to the regulation by the international lighting committee CIE, for the light distribution measurement of the discharge lamp, the probe’s V (λ) matching error f1’ should be no more than 2%.
To make the spectral response matching of the detector is in accord with V (λ) curve, we usually use a group of optical filter of different materials to add in front of the silicon light pool. It is not easy to achieve the accuracy that f1’ is less than 2％. At present the photometric silicon photoelectric detector applied in the goniophotometer, the sensitivity will reduce with the increases of the temperature. When the temperature changes 1 ℃, it will cause about 0.1% change of the sensitivity. In addition, the multiplying power of the optical amplifier circuit available will be influenced by the temperature. So we have to make the temperature of photometric detector and circuit constant, the temperature should be controlled in less than 1 ℃. If possible, we can use the rapid spectral spectrophotometer with high accuracy to replace the traditional photometric detector, so that it can eliminate the probe V (λ) matching problems, and also can be used to test the space and color distribution of LED, to realize IESNA-LM-79 LED color measurement method.
c) Angle accuracy
Goniophotometer tests the photometric data of the lamp in each direction, has higher requirement for the angle accuracy of the rotation and positioning accuracy, including the angle accuracy, axis precision, reflector face form precision ect. The flatness of the reflector is very important for the goniophotometer with the reflector. Face form error and angle installation error will affect angle accuracy; Face form error will also affect the measurement of the beam space characteristics, arouse more measurement errors. If in the measurement process the probe measurement beam axis needs to make on space rotating (surface) movement relative to the reflector, so the face shape error of the reflector will arouse four times impact on the angle precision.
d) Stray light
Stray light is one of the most important factors in the distribution photometric measurement that impact the test precision. Enough attention should be paid in the purchase and laboratory building of the distribution measuring device. It should be noted that, any black surface exists several percent optical reflection. The influence of the stray light for measuring the narrow beam was especially obvious; for example, if the project-light lamp beams Angle is 4°, even if the reflex of the environment is only 1%, the influence of the background stray light will cause the error of the total flux ( Flux Meter ) in about 40% of the above. Therefore, distribution photoelectric detectors should be monitored to receive lamps shiny surface only or the beam of light reflected from the reflector only, other stray light, such as reflector edge, ground, metope and tray light caused by reflection should be eliminated.