Check a commercial room in two conditions: bright daylight, and low or no daylight. Compare the same tasks in the same spots during the hours people actually use them. A room that looks well lit at noon may not show what the electric lighting has to do after dark.
This matters most for commercial daylight lighting in rooms used into the evening. It also matters where windows reach some desks, counters, or workbenches more than others. In both cases, daylight is one input that changes by area and by hour. It is not a fixed baseline.
How Daylight Changes the Lighting Assessment
Daylight changes how much of each task's light comes from the windows and how much must come from fixtures. So the review should start with the work, not with how bright the room feels overall.
First, list the actual tasks. That might be a front counter, a row of desks, a packing table, or a display wall. Note where each one sits and when people use it. A reception desk staffed until 8 p.m. needs a different review than a storage corner used only at midday.
Next, keep the comparison tied to those same tasks. Look at each task area when daylight is strong. Then look at it again when daylight is low or gone. The difference between the two views shows you how much daylight adds to that spot. It also shows what the electric lighting must carry on its own.
The Whole Building Design Guide advises designers to make sure natural and electric light together provide adequate light for the required task. That is the key point for a day-and-night review. Adequacy is judged at the task, using the light that is really there at that time. After dark, the window share drops away. A task that seemed fine during the day may need its electric lighting checked as if daylight were not there at all.
Map Daylight by Task Area
Daylight does not reach every part of a room equally, so assess it one task area at a time. A window-side desk and an interior desk in the same room can need different answers.
This affects how the electric lighting is laid out. WBDG guidance says fixtures should be circuited in zones that relate to the predominant daylight distribution in the space. In practice, your area-by-area notes become the starting map for those zones. Use what you see in your own room rather than a fixed distance from the window, since layout, window size, and coverings all shape where daylight lands.
| Observed condition | Planning implication |
|---|---|
| Task area gets noticeable daylight | Record its location; assess daylight and electric light together |
| Interior task area gets less daylight | Assess it on its own; the window-side area does not represent it |
| Direct sun reaches an occupied task | Include glare in the observation, not just brightness |
For the last row, more sun on a work surface is not automatically a better task condition. U.S. Department of Energy guidance notes that good daylighting balances enough daylight against too much sunlight, because direct sunlight can cause glare and excessive heat gain. A bright spot with glare on a screen or counter is a problem to flag, not a sign that the area needs less attention.
If your notes show areas with clearly different daylight, the room may need more than one lighting approach. Our guide to commercial lighting zone planning walks through how separate zones can lead to different fixture decisions in a small facility.
Turn the Comparison Into a Design Review
The next step is a short written record that a designer, contractor, or your own team can act on. Keep one entry per task area:
- Task: what people do there, such as checkout, desk work, or assembly.
- Location: where it sits, such as near the east windows or in the interior.
- Occupied period: when it is in use, including any hours after dark.
- Bright-daylight condition: what the area looks like with strong daylight.
- Low- or no-daylight condition: what it looks like with only electric light.
- Direct sun or glare: where it shows up and what task it affects.
Once the entries are done, sort them into two groups. One group holds what you saw directly. The other holds open questions, such as "Is this interior desk lit well enough after 6 p.m.?" or "Will dimming near the windows leave the counter too dark on a cloudy afternoon?" The record is a planning aid. It is not a measurement standard or an engineering calculation, so open questions should stay open until someone checks them for your project.
For questions about light levels, how lights should dim or switch, or glare, simulation is a supported next step. WBDG describes daylighting simulation that predicts illuminance, dimming and switching response, and glare probability for a given building and climate. To prepare, see our article on requesting a free light simulation, which covers the building information to gather. You can also bring your task-area notes into our online lighting layout tool when you start placing fixtures. After the review shows which zones carry after-dark tasks, you can compare fixture options for those zones in our indoor lighting collection.
FAQs
Can a day-and-night walk-through tell me how much light a task needs?
No. A walk-through shows where conditions differ and which areas depend on electric light after dark. It does not set a target light level for a task. Target levels and predicted light on the work surface belong in project-specific design, where simulation or calculation can be used for your building.
Do daylight sensors make this review unnecessary?
No. Sensors and dimming controls respond to available daylight, but someone still has to decide which task areas need support and at what hours. Your day-and-night record is what tells a designer where controls might help. If you are weighing controls, our overview of daylight harvesting with sensors explains sensor placement and dimming basics. Check the specific product and control details for your project before you buy.
Does a commercial daylight lighting review settle code requirements?
It does not. Energy and lighting code requirements depend on the project's jurisdiction and scope. A day-and-night record is useful input, but compliance calls belong to a lighting designer, engineer, or the local authority having jurisdiction. Any electrical changes that follow should be handled by a licensed electrician.
What if the room is only used during daylight hours?
Then the after-dark check matters less, because no one is using the tasks at night. You still need to look at the low-daylight periods that fall inside working hours, such as early mornings or late afternoons. If the schedule may later stretch into the evening, add those hours to the record now. That way the plan doesn't have to be redone.
