Lighting design is usually best left to a qualified lighting engineer; but, there are times that the security practitioner needs to understand lighting requirements as they relate to security, and where those needs may be at odds with the more generic illumination that the lighting engineer will propose.
This article will not make a lighting expert of a security practitioner, but it is intended to provide a primer to be able to discuss lighting design and its elements. The needs of security lighting are two-fold: first, for general illumination to enable security forces and user populations to interact effectively in their environments; and second, to optimize the performance of security video applications. This article provides the basic elements of lighting technology and identifies areas of video performance that are crucially affected by lighting.
Properties of Light
Light is electromagnetic energy within a narrow band of the full electromagnetic spectrum that stretches, as illustrated below, from gamma rays to radio waves and sound. Visible light is the band of electromagnetic energy that impacts our eyes and is known as our spectral response. Other senses (e.g., our ears), other sensors (e.g. motion detectors and video cameras) and other forms of life (e.g., plants and bats) respond to different wave bands, e.g., ultra-violet, infrared, microwave and sound.
When we see a red shirt or green grass in natural sunlight, we are seeing that part of the light that is reflected by the shirt or grass into our eyes - any other components of the electromagnetic energy that impact the object are not sensed by our eyes or are absorbed by the object. A video camera imaging sensor mimics the eye but may be tuned to react to other wavelengths, in particular the infrared band, which the human eye cannot sense.
Our sun pours energy on us that covers the complete electromagnetic spectrum - so the white, natural sunlight that we see is a mix of all the wavelengths in the visible spectrum band, the colors of the rainbow. If the light source emits energy that is visible to us but does not cover the complete visible spectrum, e.g., the very yellow light from a low-pressure sodium bulb, then the color of light that we perceive is reflected from the object is distorted and the red shirt looks green.
For some safety applications (e.g. parking lot lighting), the accuracy of the color of the car that is about to hit us is not utmost in importance - perceiving its presence, size and direction of motion are all that we care about. However, for security applications, the accuracy of the color of an object that we see is important in our ability to recognize and identify a person and their possible intent through facial expression and body language.
The accuracy of color reproduction for a particular light source is called its Color Rendition Index (CRI) and is measured on a scale of 1 to 100. A CRI of 100 (daylight) is considered best, with a CRI of 80 to 100 being excellent and a CDI of 70 to 80 is good. The table below shows typical CRIs for different light sources.
- Daylight 100
- Incandescent Bulb 100
- Halogen Bulb 100
- Fluorescent Bulb 75-100
- LED 70-100
- Metal Halide 70
- Mercury Vapor 50
- High Pressure Sodium 20
- Low Pressure Sodium 5
Another property of artificial light that is important is color temperature. This imparts a particular mood to the human observer. It is also important for the optimization of camera performance although cameras can correct for color temperature using white balance. Color temperatures are quoted on the absolute Celsius scale in Kelvin (K) (0K being absolute zero, the lowest temperature possible, 273K being freezing water, and 373 being boiling water) and are characterized as follows:
- 3,000K - Warm: friendly and intimate (restaurants and hotel lobbies)
- 3,500K - Neutral: friendly and inviting (showrooms and offices)
- 4,100K - Cool: neat, clean and efficient (classrooms and hospitals)
- 5,000K - Daylight: bright, alert, exact coloration (galleries or medical exam areas)