Light Source & Optical Measurement Technology

Editor : Sapien

So don’t be afraid to let them show your true colors.
True colors are beautiful like a rainbow.

– From True Colors, sung by Cyndi Lauper

 McScience’s test systems employ various optical measurement technologies. Many types of devices under test such as display or photovoltaic devices for instance, are involving optical characteristics and require optical measurement as an essential test part. There are experimental setups to measure ordinary optical characteristics like spectroscopy and the setups can include optical imaging systems for visual or internal inspection by images.

 Along with the electrical case, the optical experiment also comprises units for stimulation and also response measurement. For optical stimulations, various types of light sources are used. The lamp systems produce white light with different spectral distribution depending on lamp constituting materials, and continuous or flash light depending on power driving method. The Laser systems, on the other hand, output beams with monochromatic light, high intensity, small beam area, and high frequency pulse forms. The LED systems have the merit of providing various optical output signals like pulse and sine wave in combination with digital power control units.

 As means of monochromatic light irradiation, laser alone or the optical band-filter combined to LED or lamp system can be used, and the monochromator system combined to white light source are useful for varying wavelength of the monochromatic light. In case of testing materials or devices under optically excited conditions, the light source system combined with bias light for optical excitation and pulse light for stimulation is used, which is called as light modulation technique. White light lamp is broadly used for the bias light, and optical chopper, pulse LED or laser, for the pulse light.

 The light feedback technique is to regulate optical output from light source system. This technique controls output of light source normally by receiving electrical feedback from optical detector which measures intensity of output light.

 Optical measurements are generally divided into radiometry and photometry. The radiometry is for measurements of light energy or photon flux, and photometry is for measurements of light intensity based on human eye perception, brightness, luminance, illuminance, and color. The optical filter and sensor combination is used for optical intensity and color measurement, and the spectrometer for measurement of optical spectrum. The color measurement is performed generally on the basis of CIE standards.

 The lighting devices like LED and OLED are characterized by the relation between electrical input, in the form of voltage or current, and optical output. So quantum efficiency is measured by means of radiometry, luminous efficiency by means of photometry. Color index, color temperature and color rendering index can be calculated from the measured spectrum of output light.

 The photovoltaic devices like solar cell require solar simulator system as standard light source for measurement of solar conversion efficiency, and quantum efficiency is obtained by measurement of incident photon to current conversion efficiency.

 Camera imaging and image processing technology is widely used for automatic defect detection in product manufacturing process. The automatic optical inspection generally measures reflective imaging of visible light range, but infrared or thermal cameras are often used for luminescence imaging or thermography. In case that periodic control of luminescence or heat generation is possible, phase lock-in technique which synchronizes the periodic control to camera imaging is applicable.