Today I want to write about the errors committed in calibration of Light boxes by almost all ISO 17025 certified labs in India , and its acceptance by the auditors.
ISO 17025 calls for very strict calibration , verification and maintenance of instruments used for testing.
I strongly beleive that more by design , users of light boxes have been made to believe that Lux value alone need to be calibrated and the Spectral Power Distribution and Color Temperature ( the last 2 terms are seldom mentioned by anyone leave alone understand it ).
The reason for this is largely lack of knowledge of the need to to measure these values not only by the Indian companies that made low cost light boxes. Not only were these people ignorant , due to their propaganda and the lack of knowledge even among dyers , Lux measurements were considered as sufficient measurement.
The choice of Lux as a parameter was in my view due to 2 reasons , the measuring instrument is cheap and easy to achieve with commercial Fl Lamps.
Further ISO 105 A01also does not say anything about the need to measure SPD and CCT , it simply says 600 Lux and above is considered as good for Grey scale assessments , another mistake here is no maximum value is mentioned, high lux values can also cause problems.
So the Light Box makers bought low cost Lux meters , that is mostly used in the film industry to measure lux at the Heroines face and started measuring the light inside the cabinet to be 600 Lux and above and started selling certified Light boxes.
The major problem is no one know what lamps were generating the 600+ Lux , and it could be anything D65, F02 F11 etc.
The auditors who were accredited by NABL ( National accreditation Board for Testing and calibration labs ) also went only by Lux values and did not bother to check the SPD or the CCT.
One very much acclaimed Auditor even dismissed me saying that SPD and CCT are not required to be measured.
So several Indian made light boxed had D55, CWF TL84 or any lamp with 600 lux and got away as calibrated light boxes.
This is the the biggest cause for debates in visual matches as well as Grey scale values.
Comments and Recommendations.
- Please note Light Boxes need to be calibrated for Lux, SPD and CCT.
- Refer to AATCC EP 9 for more details on what must be CCT of the lamps and the desirable Lux values.
- CIE has full details of the SPD of all the lamps
- Calibration has to be carried out every 1000 hours of operation or 1 year which ever is earlier.
- Fluorescent Lamps cannot be adjusted and need to replaced with new ones if the required values are not achieved.
- Do not be taken in by price, compliance to standards and specification are more important, as Color is the biggest property based on which the Apparel business is done. Right from design to purchase of a product by a consumer.
- Good Light boxes show the color as close as it is in its original state and there by reduces debates.
- Clean the Tubes daily, or fiber keeps accumulating on the tubes and over a period of time changes the Lux , CCT and SPD .
- Paint the Light Box Munsell N5 or N7 as per the manufacturers instructions.
This is so very important to understand! Thank you for making this public. Even when proper calibration is done annually for SPD and CT, checking lux (or footcandles) monthly is a good idea to watch for declining strength of the lighting. Even then, a bulb may lose proper SPD or CT, while it can still have adequate lux. I found one just last week during a plant audit. I could tell the CWF lighting looked too yellow to me (I have been looking at color too many years at many sites!), and sure enough even though the light box had adequate lux, we replaced the bulb and could clearly see the difference in the bluer cast of the lighting.
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