Light sensors, also known as lux sensors, are widely used in a variety of lighting control applications. From a practical, or application, standpoint they fall into two types:
From a constructional standpoint, light level sensors can be either stand-alone devices or they can be integrated with a microwave or passive infrared occupancy sensor into a single unit.
The component at the heart of most light level sensors used for lighting control applications is a photo-diode. For lighting control applications, photo-diodes are made to respond to the same wavelengths as the human eye and their electrical output (measured in μA) will usually be proportionate – as the lux level in the environment increases so does the output from the photo-diode. The accompanying circuitry and software will convert the output of the photo-diode to a binary (on/off), analogue (1-10V) or digital (DALI) signal depending on the application.
In the past, light dependant resistors (LDRs) were used for light level sensing. Because they contain cadmium sulphide which is banned for most uses under the RoHS directive, they are now rarely used.
The most commonly used stand-alone light level sensor is the type seen on street lights.
A lux (light level) sensor, designed to be installed on a street light. Facing upwards, it detects the level of natural light, uninfluenced by the level of artificial light being reflected up from the street below.
Typically, this device will switch the street light on at an ambient light level of 20 lux (dusk) and off at 80 lux (dawn), though other values can be used.
This mini-photocell performs the same function as the larger unit above, but is designed for installation in floodlights and other smaller units.
This lux sensor is designed to be fitted to the outside of a building. Some of these sensors give a variable output and are used for daylight dimming (or daylight harvesting) applications, controlling the lighting inside the building.
In such instances, as the natural light level increases the output from the sensor also changes, signalling to the lighting control system that the interior fittings should be dimmed down.
Light level sensors are very often incorporated in occupancy sensors. These may have two functions:
A “maintained light level” is a lighting control technique used in indoor areas such as offices and classrooms. A light level sensor located in the area being controlled directly monitors the actual light level and compares it to a target light level that has been set. If the actual light level is measured to be too high or too low the sensor issues a command to the associated fittings instructing them to dim down or get brighter so that the actual light level is adjusted to come closer to the target light level. This is a closed loop system.
A PIR occupancy sensor also incorporating a lux (light level) sensor. This could be used to implement a maintained light level.
In theory, a maintained light level should have several benefits:
In practice there are several difficulties that can be encountered.
A lux sensor located in a ceiling is not measuring this. It is measuring the light that is being reflected from the working plane, not the amount of light that is incident to it. Consequently, any change to the surface that is reflecting light back to the sensor will change the sensor’s measure of the light level, even if the actual lux level has not changed at all.
For example, 500 lux might be the target light level for the working plane in an office filled with light grey desks. If I leave a large sheet of white paper on the desk beneath the sensor it will see more reflected light and interpret that to mean that the target of 500 lux has been exceeded and therefore dim the lights. Conversely, if I sat at the desk wearing a dark jacket the sensor would see this as a dim environment and issue a command for the lights to be made brighter. A similar effect will be noticed if the furniture is moved or the carpet is changed.
One solution to all these problems is to use another technique, daylight dimming (or daylight harvesting) instead.
“Daylight dimming” (also known as “daylight linking”) is an open-loop lighting control technique. With daylight dimming the light level sensor is located outside the building where it will not be affected by the lighting inside. As the natural light level outside increases the sensor issues a command for the artificial lights inside to dim down, and vice versa.
This technique avoids all the problems associated with a maintained light level.