IEC TS 60904-13-2018 pdf free.Photovoltaic devices – Part 13: Electroluminescence of photovoltaic modules.
The camera response function (CRF) relates the actual quantity of light impinging on eachelement of the sensor array to the pixel values that the camera outputs. When the sameobject is captured at different exposure times but with an otherwise identical camera setup, anon-linear CRF causes the resulting image intensity distribution to exhibit nonlinearity, evenafter application of a correction for exposure time. Therefore,when analyzing imageintensities,either the linearity of the CRF needs to be assured (basic methods are commonlyfound in the camera literature), or a correction for non-linearity needs to be performed usingimage processing. Scientific grade Si or lnGaAs-based sensors are often linear or have acorrection for non-linearity embedded. Neglecting non-linearity will cause erroneous resultswhen applying procedures for image correction that are given in Annex A or any quantitativeanalysis.
To obtain maximum image resolution and electroluminescence signal, the optical axis of thecamera is placed perpendicularly and as close as possible to the module face to image thesolar cell or module area. lmages captured at the highest resolution may require a longerexposure time and time to transfer from the camera and process.Binning features may existto combine pixels for lower resolution and shorter image processing times. Gain feature mayexist to amplify the signal of the EL image.
IEC TS 60904-13 Lenses shall be free of absorption filters or coatings that remove the infrared near the band-gap of the semiconductor material to be examined. Optical glass is generally suitable,however Ge lenses will be necessary for measuring EL from the very low band gap materials(under 0,6 eV).Lenses vary from telephoto to wide-angle in focal length. Choice will dependon the specific application and geometric considerations when capturing the image. Wide-angle lenses that have short focal lengths used in conjunction with the higher resolutioncameras capture a larger FOV.The camera may be placed much closer to the subject,whichis useful when space is constrained.Some wide-angle lens optics however cause undesirablebarrel distortion in the images that will require correction by post-processing. Lenses withlonger focal lengths generally have less barrel distortion and can therefore more accuratelyimage a module,whereby the resulting images may require little or no correction by postprocessing.Lenses may feature components that correct for the difference between visibleand infrared wavelengths, which can aid in focusing.
Filters on the camera lens may be used to help cut light of extraneous wavelengths frombeing detected.850 nm to 950 nm long-pass filters may be used when imaging near band-edge EL from modules with silicon cells.
4.1.2Dark room imaging studio or environment
A darkened environment is favored for high quality images.Precautions should be taken toeliminate stray light entering the imaging studio,such as with use of hard walls,curtains,baffles,and sealing of any gaps with material that are of light absorbing nature (black). lf afilter is used on the camera,then LED lighting may be used that emits light only in thespectrum that is cut by the filter.For non-laboratory measurements, minimize extraneous lightwhen possible.For example,perform measurements at night. lf stray light is present, animage subtraction procedure will be required,as discussed in TS 60904-13 pdf download.