Light Level Sensors

INTRODUCTION

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:

• Light sensors for on/off control. These will have a binary output. Their function is to turn lights on or off.
• Light sensors for on/off and dimming control. These will have a variable output which can be either of two types.
• A variable voltage. Most commonly the voltage output will be variable between 1-10V. The output voltage is inversely proportional to the light level detected, meaning that when the sensor detects a high lux level the voltage output will be low and when it detects a low light level the output voltage will be high. This is commonly known as an analogue control signal – the output voltage is (inversely) analogous to the light level that has been detected.
• A digital signal. The most common digital signal protocol in use now (2021) is DALI. A DALI enabled sensor will output a digital signal onto the DALI bus that indicates to the application controller the light level that has been detected and the identity (or address) of the sensor involved. The application controller will then generate appropriate DALI commands to instruct the affected lighting groups to get brighter or dimmer. For more information on DALI, please read "What is DALI ?" article.

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.

HOW DOES A LIGHT LEVEL SENSOR WORK?

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.

STAND-ALONE LIGHT SENSORS

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 SENSORS INTEGRATED WITH OCCUPANCY (PIR OR MW) SENSORS

Light level sensors are very often incorporated in occupancy sensors. These may have two functions:

• To provide a switch-on light level threshold. If occupancy is detected and the local light level is below the threshold that has been set then the lights will switch on. However, if the local light level is above this threshold then the lights will remain off. This lighting control technique is practical, reliable and widely used.
• To provide dimming control – maintained light level. In this application, the light level sensor is part of a closed loop feedback system. If the local light level, as measured by the sensor, rises past a target level the sensor sends a signal (typically using DALI) to the connected light fittings to dim down, and if they dim below a target level the sensor generates a signal causing the light fittings to brighten. In theory, this is an effective energy saving idea, but in practice it can be difficult to implement.

WHAT IS A MAINTAINED LIGHT LEVEL?

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:

• It saves energy. If there is plenty of natural light available then dimming the electric lights is an energy saving measure that does not compromise the working environment in the office or classroom involved.
• It is simple to install. The light level sensor is typically integrated with an occupancy sensor, so one device can perform two functions.

 In practice there are several difficulties that can be encountered.

• Hunting.This is a repeated or continuous fluctuation of the output of the electric lights caused by the sensor being incorrectly calibrated or wrongly located. What happens is this: the sensor detects that the light level is higher than the target level and dims the lights down to compensate. The dimming goes too far, the sensor attempts to correct the situation and the light level increases but overshoots the target. Potentially, this situation can repeat itself endlessly. Usually this can be corrected in the sensor’s software and leading manufacturers have all learned how to overcome this problem.
• Lighting overlap. If a sensor is influenced by lighting from an adjacent zone a problem similar to hunting can occur. If the lighting is zone A dims down, but the sensor in zone B detects the change, the sensor in zone B will issue a “get brighter” command. The lighting in zone B then brightens, but if this is detected by the sensor in Zone C or A a ripple effect is created with zones brightening and dimming in what seems to be a never ending but random sequence. An important rule of maintained light levels is to ensure that each sensor is only detecting the lights that it controls.
• Furniture and task changes. Lighting standards such as EN12464 recommend light (lux) levels for different tasks. To check if these are being achieved one would use a light meter, place it on the working plane facing upwards and take a reading. The meter is measuring the amount of light that is incident on the working plane.

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.

WHAT IS DAYLIGHT DIMMING?

“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.