To obtain accurate photo metric data, the following should be noted:
1. Measuring the photometric luminous intensity is by measuring illuminance with a certain distance, and then calculate it to obtain the luminous intensity value according to the distance inverse square law of photometry, which is IE R2. I stand for the intensity in the test direction, E stand for illuminance of the receiving surface of photoelectric detector, R stand for test distance. But for many lamps, photometric law of the near field is not applicable especially for LED lighting, CIE file clearly defined for this, the test distance of luminaire photometric should be large enough (CIE Technical Report _The Measurement of Absolute Luminous Intensity Distributions[CIE Pub.NO.70]), meet following conditions:
For fluorescent type lamps: R>D×10
For project type lamps: R>D×200/
2. Accuracy of Photometric
Photometric detector is an important part of the goniophotometer, the spectral response of the detector S (λ) accuracy should be in line with the photopic spectral luminous efficiency function of human eye V (λ), which is S (λ) = V (λ). According to the regulations of the International Commission on Illumination (CIE), for the measurement of the light intensity distribution of gas discharge lamp, the matching error f1’ of the detector V (λ) should not exceed 2%. To match the spectral response of the detector with V (λ) curve, we usually use a set of filters of different materials, and add it in front of silicon photonic pool. Due to the limit of spectral transmittance curve of the glass material, to achieve the accuracy of f1 ‘ less than 2% is not easy. Currently, the sensitivity of silicon photo detector which applied in goniophotometer decreases with temperature increasing, temperature changes about 1 ℃, probably will cause 0.1% changing of the sensitivity. Besides, photocurrent amplifier circuit magnification also affected by temperature. In this case, it is necessary to keep constant temperature for the photometric probe and the circuit, which should be controlled within 1 ℃. If conditions permitting, the high-precision CCD spectroradiometer can replace the traditional photometric probe, thus can eliminate the problem of V (λ) matching of the probe, which also can be used to test LED spatial color distribution, to achieve LED spatial CCT measurement method of IESNA-LM -79.
3. Accuracy of Angle
Goniophotometers is used to measure photometric data of luminaire in all directions. It has higher requirements for the accuracy of angle of its rotation and positioning system, which including the accuracy of angle, axis and reflective mirror shape. For the goniophotometer with mirror, mirror flatness is very important, mirror surface error and angle installation error will affect the precision of angle as twice; Mirror surface error will also affect measuring spatial properties of the beam, causing greater measurement error. During the testing, if the measuring beam’s axis of the detector needs to do space rotation (cone) movement relatively to reflective mirror, then the mirror surface error will affect the angle accuracy as quadruple.
4. Stray Light
Stray light is one of the most important factors that affect the test accuracy during the luminous intensity distribution measurement. We should take it seriously enough for purchasing and lab establishing of the equipment for luminous intensity distribution measurement. It should be noted that there also exists a few percent of the optical reflection on any black surface. The effect of stray light is particularly obvious on the measurement of narrow beam lamps. For example, if the floodlight beam angle is 4°, the influence of background stray light will cause 40% or more error of total flux even if the reflectance of environment is only 1%. Thus, the goniophotometer’s photo detector should only receive the light-emitting surface of luminaries or the beam reflected by the reflective mirror. Other stray light should be eliminated, such as reflective mirror edge, ground, walls and other reflections.