FAQ: “I want to measure the lux level for a light box we are using to evaluate the color of garments. I would like to know, how many light boxes are required for a room. One light box has four D65 tube rods. My question, Is there any standard value of lux which we need to maintain and how do we find out for that lux level? How many light boxes will be enough.”
The standard illumination on the floor of a light booth should be between 810 and 1880 lux as defined in ASTM D1729 Standard Practice for Visual Appraisal of Colors and Color Differences of Diffusely-Illuminated Opaque Material and identified in Section 6.1.2.“6.1.2 Photometric Conditions—For critical evaluation of color differences of materials of medium lightness, the illumination at the center of the viewed area shall be 1080 to 1340 lx (100 to 125 fc). For general evaluation of materials of medium lightness, the illumination shall be between 810 and 1880 lx, (75 and 175 fc). In either case, for viewing very light materials, the illumination may be as low as 540 lx (50 fc), and for viewing very dark materials it may be as high as 2150 lx (200 fc). This higher level of illumination is usually obtained by holding the specimens nearer the source.”
If you want to measure the lux level in a light booth, you should purchase a lux meter (search the Internet for “lux meter” and you will find many).
The luminous emittance of a lamp in a light box will drop in power over time, as much as 50%. Typically the lamps are standard 4 foot D65 fluorescent tubes that can be replaced. Replacement of the lamps should bring a light booth back into conformance in terms of amount of power as with the lux power requirement of 810 to 1880 lux is quite broad.
If you are going beyond just a single light box to make a an entire room into the correct D65 illumination and corresponding lux level, typically what you do is make the whole room into one giant light box by replacing all the fluorescent lamps in the room lighting fixtures with D65 balanced lamps; blocking off any light coming through windows or doors; and painting the walls with a neutral gray Munsell N5 or N7 paint depending on whether you are evaluating darker or lighter colors.
HunterLab does not manufacture light booths but a company you could contact regarding this is:
FAQ: “I am a little confused on the distinction between D65 and lux. Can you explain?”
“D65” refers to a CIE definition of the spectral quality or output of a specific white daylight at noon on a sunny day. At sunrise and sunset, daylight is much warmer, typically around 5000 kelvin. At mid-morning and mid-afternoon it is cooler or less warm, rated at 5500 K. Noon daylight rated 6500 K and overcast at 7500 K, moving from warm daylight to cool daylight.
A measurement in lux is the intensity of the light expressed in absolute lux units. D65 is a standardized measurement of the spectral quality of light; lux is a measurement of the intensity of the light.