Solid State Lighting Assessment Technical Information
Visible light is a form of electromagnetic energy, part of a spectrum that includes radio waves, X-rays, ultraviolet and infrared. Visible radiation is commonly called “light”, and is usually described by its wavelength, which is expressed in nanometers. One nanometer is one billionth of a meter, or roughly four ten-billionths of an inch. The relationship of light to other forms of energy is illustrated below. The human eye can only see a part of this energy spectrum – a very narrow band of wavelengths between about 380 and 780 nm. The hue we see as blue lies below 480 nm, green between approximately 480 and 560 nm, yellow between 560 and 590 nm, orange between 590 and 630 nm, and red appears at wavelengths above 630 nm.
Sources of light such as the sun, florescent lamps, tungsten-filament light bulbs and LEDs emit light that is composed of a combination of visible wavelengths. A curve showing the amount of each of the visible wavelengths emitted by a light source is known as its “spectral power distribution”. The hues we see of physical objects are directly affected by the spectral power distribution of their illuminating light sources. And more importantly for motion picture production, the hues seen by film stocks and digital motion picture cameras are also directly affected by the illuminant’s spectral power distribution in distinctly different ways than the human eye. Links to video sequences illustrating the effect on skin tone, makeup, wardrobe and props can be found on the Solid State Lighting main page by clicking here: Main Page
Standard Illuminant Curves
The curves shown below are the spectral power distributions for three common references often used as a basis for comparison with actual lighting instruments. They are standard illuminants defined by either the International Commission on Illumination (CIE) or the International Standards Organization (ISO). The first curve – labeled “CIE E” – represents a theoretical source that has equal energy at all wavelengths. The second curve – labeled “CIE D55” – represents mid-morning or mid-afternoon daylight and has a correlated color temperature of approximately 5500 degrees Kelvin. The third curve – labeled “ISO Studio Tungsten” – represents the light emitted by tungsten filament light bulbs.
Sample of Typical Lighting Instruments
The following curves represent typical LED lighting instruments currently on the market. The LED-based instrument curves are very different than the references curves, and therefore the hues of objects illuminated by them are noticeably different relative to the reference curves and to each other.
The lighting instrument represented below is constructed from what appears to the human eye to be a white LED, but in fact it emits a narrow band of blue light that excites a broad yellow phosphor. When the light from the yellow phosphor is combined with the blue emission, the result is a light that appears to be white.
A similar approach to producing visible white light is the “blended phosphor” LED that uses a mix of phosphors with the blue emitter to create a curve that is more like Studio tungsten lights.
Another type of instrument uses multiple LEDs that each emit light at different wavelengths. The simplest form of a multi-emitter instrument is a “tri-color panel” using red, green and blue LEDs.
A variation of the multi-emitter technique uses additional LEDs that emit additional wavelengths to create a closer match to Studio Tungsten lighting, although the resultant curve is “spikey”. This has noticeable side effects in motion picture production, which are discussed below.
The Visible Effect of Sample Lighting Instruments
Even though a lighting instrument produces what appears as white light to the human eye (or to film or a digital camera’s sensor), the hue of an object being illuminated by “apparent white light” can be drastically different than expected. A common test chart used for assessing color performance of motion picture imaging systems is the Macbeth chart, which consists of a number of patches of well-specified colors and gray shades. The charts below are called “split Macbeth charts” because each color patch shows the visible effects of two light sources – studio tungsten in the top half of the patch, and an LED lighting instrument in the bottom half. Each chart below corresponds to the light sources described by the spectral power distributions above. Although your computer display is not likely to be a calibrated reference monitor, the wide variations in color patch hue caused by different illuminant spectral power distributions should be readily apparent.