Categories Lighting

The Difficulty with Daylight Hold-Off Controls

The call for improved energy efficiency and government/energy code requirements are driving a need for greater use of lighting control. One such example is the new federal requirement that all states adopt building codes that are at least as stringent as the most recent American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) building code (90.1-2010) by October 2013.� That code requires automatic shutoff and dimming in many space applications, and will encourage numerous construction projects to go beyond that by connecting lighting systems to building management systems.

So, reducing electric lighting loads when daylighting exists makes sense, however, implementing this concept with the introduction of daylighting controls is not quite as simple. First, let’s take a look at the two types of daylighting systems; closed-loop and open-loop:

Open Loop: The photo sensor does not respond to, or “see”, the electric light that it controls.

Closed Loop: The photo sensor senses and responds to the electric light that it controls.


Closed loop systems are generally preferred since they adjust for any changes in the space and react and adjust to what is actually happening. For example, in a parking garage application one could have a situation where a series of daylight openings are blocked by large vehicles or a construction project where significantly less light is contributing to the space; an open-loop system would recognize the sun is out and dim the luminaires (incorrectly), whereas a closed-loop system would recognize the space is actually still very dark and would keep the fixtures at higher power to compensate for the blocked daylighting.

Open-Loop and Closed-Loop Sensors – Where Do We Put Them?
The difficulty with closed-loop daylighting controls is now we are asking the control system to understand how much of the daytime lighting contribution is from the luminaires, and how much the contribution is from the sun.

The challenges with open-loop systems are that the sensors are typically remote from the fixtures, and the system cannot account for variations in light. Many controls manufacturers have side-stepped this issue by utilizing an open-loop approach where the sensor resides in the luminaire, this is referred to as “Daylight Hold Off.”

Wattstopper, a lighting controls company, refers to this approach in their installation sheets as “Light Level Feature,”� which holds OFF and turns OFF lights when the space is occupied and adequate ambient light exists. When the light level drops below the set level it will turn the lights ON when the space is occupied. The factory setting is for maximum, meaning that even bright ambient light will not cause the sensor to hold the lights OFF if it detects occupancy in the controlled area. 

We know that open-loop systems measure the daylight contribution and alter the luminaire light output based on the readings from the photo sensor. Normally, an open-loop system has a sensor mounted so it’s in direct view of the daylight (see figure above), in other words remote from the luminaire. But in this case the photo sensor is in the fixture, how would that work?

Short Answer: Not very well.

Long Answer: An open-loop system must only read the daylight contribution in the space, so in order to accomplish this task while the sensor resides in the luminaire the sensor must wait until the luminaire is shut-off before the device can take measurements. At that point, when the luminaires are off, the sensor will take a reading and determine if the daylight contribution exceeds the user selected light level on the control — if the light level is, in fact, exceeded. Once the space is re-occupied, the system will hold the lights off, or in a lower level state.

If there is enough activity so the lights don’t shut-off, then this type of open-loop daylight control is rendered completely ineffective. An ideal application for hold-off controls would be in a small office. Conversely, in a space that has a lot of foot or vehicular traffic, this would be a poor application because it is conceivable, and in many cases likely, that the luminaires would not dim at all during the entire day.

The difficulty with closed-loop controls is the commissioning of the luminaires and the controls. Many closed-loop controls systems require that the controls are initially turned-on during the night so that the levels being read are only electric. While this is effective, it is sometimes very difficult to manage and can be costly because highly-paid technicians would need to attend the location after-hours.

The ideal solution is to have controls that work on the closed-loop principal – always measuring both electric and daylight levels simultaneously. Utilizing controls that can be commissioned for light level without technicians present after-hours provides the best scenario of easy and cost effective commissioning with accurate daylight harvesting control.