MH Controls Corner: Automatic Daylight Harvesting – Open Loop vs. Closed Loop Photo Sensing

A significant addition to the lighting controls requirements in IECC 2015, compared to previous codes, is automatic daylight harvesting. Natural daylight coming through the windows or skylights in a space is utilized to reduce the electrical or artificial lighting in a space to minimize the amount of lighting energy used.

There are many considerations that must be made to design a system that automatically reduces artificial light levels based on daylight. The intent is to ensure a correctly-operating system that maximizes energy savings, while minimizing the impact on occupants of the space. Automatic daylight harvesting can be accomplished by continuously dimming or switching fixtures on and off. However in the IECC 2015, there are particular spaces, such as classrooms, that require continuous dimming. For either switching or dimming, there are two basic categories of daylight sensors – open loop and closed loop.

Open Loop Daylight Sensors

Open loop sensors work by measuring the natural light level and adjusting the artificial lighting accordingly. It is critical that these sensors measure only the natural light with no artificial light component. If the sensor’s readings include both natural and artificial light, the calculations will be skewed and will reduce the effectiveness of the system. In some cases, sensing both artificial and natural light could result in light level cycling where the light level in the room will cycle up and down.

For optimal daylight harvesting, the placement of the sensor is crucial. The sensor needs to be in the space where the fixtures are located and aimed to only detect the light outside of a window. If the sensor is placed outside of the space, natural light level changes due to the closing of window shades will not be compensated for, and could result in low light situations.

The advantage of using an open loop daylight sensor is that a single sensor can be used to control multiple zones of lighting, each calibrated to dim differently, based on the location of the lighting zone in the space.

Drawbacks of open loop sensors include:

• Complexity of programming – calibration of the interior light must be done manually in relation to the exterior light. It can be challenging to consistently achieve the desired light level in all conditions.
• Seasonal variations in the angle of the sun will create differences in the light level sensed, and will create variations in the light level in the space.
• With the sensors placed inside the space, tinted windows and light shelves can present significant challenges and may even make the sensor read too low of a light level in all conditions to be functional.

Closed Loop Daylight Sensors

Closed loop daylight sensors are designed to measure both the natural and artificial components of light in a space. These are typically ceiling mounted and directed straight down, measuring the light level on the surface below and adjusting the level of artificial light to maintain the target light level. Placement of this type of sensor is crucial as well. The location of the light sensor needs to take into consideration the reflectance of the surface below and has to be close to the lights controlled. With indirect lighting, considerations need to be made to be sure the indirect component of light is not directed into the sensor.

The main advantages of using a closed loop photo sensor are the accuracy of light reading and the ease of configuration to achieve the target light level. Easier configuration leads to more reliable operation in practice. These sensors will operate more constantly in spite of seasonal sunlight changes, tinted windows, or sun shelves located outside of the window.

Drawbacks of closed loop sensors include:

• A separate sensor must be used for each zone of lighting to be controlled. Since the sensor measures the light in a specific area, they can only control that area.
• Altering the color or reflectance of the surface below the sensor can impact the light level measured by the sensor. Dark carpet can make the senor more difficult to calibrate. Furthermore, if the sensor is located above a work surface, items placed on that surface can impact the light level in a room. For example, if the surface is typically white, a black object (such as a laptop computer) placed on the surface will impact the light level in the space.

Summary

In general, for spaces requiring a single zone of light fixtures to be controlled, a closed loop sensor is recommended. This recommendation is based on the collective experience that MH Controls has gained after installing and commissioning thousands of daylight sensors. For projects with more than one zone of lighting control, it is preferred that each zone have its own closed loop sensor. However, this approach must be balanced with cost considerations.

For both sensor types, sensor placement is critical. The overall design of the space, including window shades, orientation to the sun, window tinting, finish colors, and furniture layout, must all be considered to design an optimally functioning system.

This article was written by Clint Conley, our Lighting Controls Department Manager. Clint can be reached at 720.904.8554 or clint@mhlighting.com.