Luminys Systems Corp. Editorial for Crash Testing Technology magazine
Many of us think of light only in terms of it’s quantitative measurements, (e.g. lumens, watts), but there are other qualitative characteristics of different light sources that can greatly effect photographic results. Color temperature, color rendering, and the light source’s physical characteristics are as important as lux when creating the optimal lighting for data analysis.
Color temperature or degrees Kelvin – Kelvin temperature describes how the color of the light relates to the color spectrum. You may have had the experience of observing the color of an article of clothing inside a store under tungsten lighting only to find the color changed outside under sunlight. That is because the quantity of blue colors available in tungsten or incandescent light are much less than the quantities available in natural sunlight. Because the store light does not provide much blue, your eyes are not able to distinguish much blue until the sunlight provides it. Most high speed video cameras are tuned to a Kelvin temperature of about 5,000 (close to natural sunlight). You can white balance your camera to accept almost any color temperature range, but as that range varies from 5,000 Kelvin, the camera is forced to either suppress dominant colors or increase gain on colors lacking in sufficient quantity. This suppression and gain balancing will end up giving you a reasonably accurate color reproduction, but it is at the expense of overall picture quality. It is well known that suppressing parts of the image and/or adding gain to parts of the image will increase camera picture noise, which of course decreases the final picture quality.
Typical Kelvin temperatures of various light sources:
3,200 +200, – 500 Tungsten or incandescent
5,400 +0, – 500 Luminys or SoftSun ESL
6,500 + 1,000, – 1,000 HMI
Color Quality or CRI – The effect light has on color is stated in terms of CRI or color rendering index. CRI is rated on a scale of 1 to 100 with 100 being best. The details of the mechanics concerning how a light source is evaluated for CRI, are quite complex. For most of us it is enough just to know that the closer a light source is to the 100 mark, the better it will recreate color and thus assist in critical detail evaluation. In some instances such as city street lights (about 60 CRI), CRI is sacrificed for higher light output efficiency for better energy efficiency. This is evident when attempting to discern accurate colors under a street light. At the other end of the scale are lights used for movie and TV production (CRI 90 and above), accurate color is so critical that they are willing to sacrifice light output efficiency for accurate color rendition. Similarly crash testing results can be best evaluated if the colors are as accurate as possible so as to not give misleading interpretations.
Soft Light and Hard Light – A light with a small point source (regardless of wattage) illuminating an object will produce shadows. Think of the sun casting a shadow from your body as you walk along. Yes, the sun is large, but because of its long distance from the earth, it acts as a small point source. A light coming from a large source such as a bounce umbrella will wrap around an object filling in shadow areas such as age lines and wrinkles on a face or nuts and bolts on an automobile assembly or the folds of an air bag as it unfolds during deployment. For example a large light source such as an overcast sky creates no shadow from your body as you walk along the sidewalk. Although hard light can help to define an object, it can also create shadows which can hide critical details such as the nature of how an airbag unfolds. A combination of the two will create an optimum photographic condition to render images with precise clarity.
Notice on image 1 how the heavy shadows hide some of the detail. The soft light on image 2 eliminates the shadows for the most part but sacrifices definition due to limited dimensional depth. The combined soft light with hard direct light on image 3 minimizes shadows while still enhancing definition by providing more three dimensional depth and realism.