Two very different devices get sold under the same search term. One is a small photocontrol that turns a parking lot light on at dusk and off at dawn. The other is a photosensor that reads light levels all day and trims the electric lighting as sun comes through the glass. They cost different money, mount in different places, wire into different systems, and satisfy different sections of the energy code.
Ordering the wrong one is a common and expensive mistake, because both are correctly called "daylight sensors" by somebody. This guide separates them, then walks through the decision that actually determines whether the installed system works: open loop or closed loop.
The two devices, plainly
A photocontrol, the thing most people mean by "photocell", is a switch. It senses ambient outdoor light and closes or opens a circuit at a set threshold. It runs exterior lighting on a dusk-to-dawn schedule. It does not dim, it does not know or care what the electric lighting is doing, and it is a single on/off decision made once at dusk and once at dawn. Photocontrols come as button types that mount through a small hole in the fixture housing, and as locking (twist-lock) types that drop into a receptacle on top of a pole-mounted or area luminaire. The locking type is standardized: ANSI C136.10 is the American National Standard covering locking-type photocontrol devices and their mating receptacles, which is why a twist-lock control from one manufacturer fits a receptacle from another.
A daylight sensor, properly a photosensor, is a control input. It measures a light level, either in the space being lit or the daylight arriving at it, and feeds that reading to a controller, which raises or lowers the electric lighting to hold a target. That adjustment is continuous on a dimming system and stepped on a multilevel one; both exist, and the difference is covered below. When the sun is on the windows, the fixtures near the glass pull down. This is daylight harvesting, and it is what energy codes mean when they require "daylight-responsive controls." It needs a dimmable or switchable driver, a control protocol such as 0-10V, and commissioning.
If the job is a parking lot, a wall pack, a canopy or a landscape run, you want a photocontrol. If the job is an office, classroom, warehouse aisle or retail floor with windows or skylights, and a plan checker is going to look at it, you want a photosensor.
Open loop or closed loop: the decision that matters most
Every daylight-responsive photosensor is one of two architectures. Getting this wrong is a common cause of systems that occupants eventually override, because a sensor in the wrong architecture reacts to conditions it cannot correctly read.
A closed-loop sensor points into the room. It sees the total light landing on the work surface: daylight plus the electric light it is itself controlling. Because the lighting it adjusts feeds back into its own reading, the loop is closed. ETC's daylight harvesting white paper puts it directly: closed-loop sensors "measure the combined contribution from both natural and the electric lighting," and the arrangement is "considered more accurate than open loop." That accuracy is why closed loop is the normal choice for spaces where the delivered light level actually matters: private offices, classrooms, open-plan office bays, anywhere with a defined task illuminance.
An open-loop sensor points at the daylight. It is aimed out a window or up at a skylight, or mounted outside altogether, and it deliberately excludes the controlled fixtures from its view. ETC describes these as sensors that "measure only the natural light, and do not consider the electric lighting contributions," typically "mounted outside of the building or near a window or skylight facing away from the controlled light fixtures." Because one reading of available daylight can drive many zones, open loop is how you control a whole daylit façade or an entire skylit warehouse bay from a single sensor.
The tradeoff is what each one cannot see. An open-loop sensor does not know the blinds are shut, so it reads bright sky and dims a room that has gone dark. A closed-loop sensor handles that correctly, because it is reading the room. But a closed-loop sensor covers one zone, so a large daylit area needs several of them.
A useful rule of thumb: closed loop for one space where the light level matters; open loop for many zones driven by one daylight condition.
Rock stocks the canonical pair from Wattstopper's DLM platform, and the model names say which is which. The LMLS-400 is the closed-loop single-zone photosensor, described by Legrand as a "closed loop photosensor that measures the ambient light level in order to automatically switch or dim one zone of lighting," and the LMLS-500 is the open-loop multi-zone unit. Lutron's wireless Radio Powr Savr daylight sensor is an open-loop device; Lutron's own submittal describes it as "daylight compensation through Lutron reliable open loop control."
Where the sensor goes
Placement is not a field decision to be made from a ladder. It is part of the design, and the manufacturer's instruction is the one that governs.
Lutron's submittal for the Radio Powr Savr daylight sensor gives an unusually clear rule: "Place the daylight sensor so the arrow is pointed at the nearest window at a distance from the window of one to two times the effective window height (H)." The submittal defines that distance in terms of the effective window height, so take H from the drawings rather than estimating it on site. That same document puts the sensor's light range at "0 to 1600 lx (0 to 150 fc)."
Beyond the specific rule, three placement errors recur:
- Aiming a closed-loop sensor at a bright surface. A sensor looking down at a white desk, a glossy floor or a light-colored partition reads that surface, not the room. Rearranged furniture then changes the calibration.
- Letting direct sun strike the sensor. A photosensor in a sunbeam saturates and holds the lights at minimum regardless of what the rest of the space is doing.
- Putting an open-loop sensor where the controlled fixtures can reach it. If the lighting it dims feeds back into the reading, it is no longer open loop, and it will hunt: dim, read darker, brighten, read brighter, repeat.
For wireless sensors, range is a separate constraint from placement. For the Radio Powr Savr models covered by the submittal cited here, Lutron specifies that local load controls sit "within 60 ft (18 m) line of sight, or 30 ft (9 m) through walls" of the sensor. Ranges differ by model within the same family, so read the figure off the submittal for the exact part number you are ordering.
Switching or continuous dimming
Photosensors do one of two things when daylight arrives.
Switching (or stepped) control cuts the electric lighting in discrete steps: full, half, off. It is cheaper and it works with non-dimming drivers, but occupants notice every step, and a visible step change is what gets a system switched to manual.
Continuous dimming ramps the output smoothly against the daylight, and occupants generally do not perceive it happening at all. It needs dimmable drivers and a dimming protocol.
A point worth being precise about, because it is widely misstated: California's Title 24 does not require continuous dimming. Section 130.1(d) requires the control to adjust lighting "via continuous dimming or the number of control steps provided by the multilevel controls". Stepped daylighting control is code-legal. The Title 24 daylighting post covers what the section does require.
So this is a specification preference, not a compliance obligation. We would still steer most projects to continuous dimming, because a visible step change is what prompts occupants to override the system, and on an LED job the drivers are frequently dimmable already. But if a budget forces stepped control, that choice does not by itself put the job out of compliance.
Wired or wireless
A wired photosensor is a low-voltage run back to a room controller or panel. It never needs a battery, it commissions on the wire, and it is the default in new construction where the ceiling is open.
A wireless photosensor removes the low-voltage pull entirely, which is why it is the common choice in retrofits: no ceiling access, no conduit, no disturbing an occupied ceiling grid. The tradeoffs are RF range and batteries. Lutron rates the Radio Powr Savr sensor for a "10-year battery life" on "one CR 2450 lithium battery," which is long enough that the battery is a maintenance line item rather than an objection.
Our wired vs wireless controls guide works through the choice in more depth, and the Wattstopper DLM vs Lutron Vive comparison covers the two platforms these sensors belong to.
When the code requires one
In California, Title 24 Part 6 requires automatic daylight-responsive controls once the general lighting in a daylit zone crosses a wattage threshold. Under the 2025 Energy Code, which took effect on 1 January 2026, that threshold is 75 watts of general lighting in any primary sidelit, secondary sidelit or skylit daylit zone, reduced from the 120 watts that applied under the 2022 code. The zone geometry, the exemptions and the acceptance testing are the substance of the requirement, and they are covered in the dedicated Title 24 daylighting requirements post and the broader Title 24 lighting controls compliance guide. Exterior work has its own rules, covered in Title 24 outdoor lighting controls.
Outside California, which document governs depends on what the jurisdiction has adopted: an edition of the IECC, or ASHRAE 90.1. Both address daylight-responsive controls, and the trigger thresholds and exemptions differ by document and by edition, so a rule of thumb carried from one state does not transfer. Our ASHRAE 90.1 vs California Title 24 comparison covers how the two differ; confirm the governing edition and any local amendments with the authority having jurisdiction before you order.
Lutron states that the Radio Powr Savr sensor "meets CA (U.S.A.) Energy Commission Title 24 requirements." Treat a manufacturer compliance claim as a starting point, not as the compliance document. The installed system, its zoning and its acceptance test are what the plan checker signs.
Commissioning is the step that gets skipped
A daylight harvesting system is not finished when the sensor is mounted. It is finished when it has been calibrated against a measured light level in the occupied space, ideally after dark to establish the electric-light baseline, and again in daylight. An uncalibrated photosensor either does nothing, or dims a room people are trying to work in, and in both cases someone eventually puts it on manual override.
Budget the commissioning visit into the job at bid time rather than discovering it at closeout. California projects should also confirm which acceptance tests apply to the controls being installed, since those are a documented part of the Title 24 submittal package rather than an optional extra.
Choosing, in short
| If the job is | Buy | Why |
|---|---|---|
| Parking lot, wall pack, canopy, pole light | Photocontrol (button or twist-lock) | Dusk-to-dawn switching, no dimming, no commissioning |
| One office, classroom or bay where the light level matters | Closed-loop photosensor, e.g. LMLS-400 | Reads the room, most accurate, single zone |
| A whole daylit façade or skylit warehouse, many zones | Open-loop photosensor, e.g. LMLS-500 | One daylight reading drives multiple zones |
| Retrofit with no ceiling access | Wireless photosensor | No low-voltage pull; check RF range and battery life |
| Anything a plan checker will review | Continuous dimming, not stepped | Stepped control is what occupants disable |
Rock Lighting and Electric stocks daylight sensors and photocontrols from Wattstopper, Lutron, RAB and Westgate, alongside the lighting controls, occupancy sensors and exterior lighting they pair with. If you would rather have the controls package priced against the drawings, send us the lighting schedule and the jurisdiction and we will quote sensors, room controllers and dimmable drivers that work together. Contractors can open a contractor account for project pricing, or contact us if you are not sure whether a space needs one sensor or six.
Related guides
- Title 24 Daylighting Control Requirements, Explained
- California Title 24 Lighting Controls: 2025 Compliance Guide
- Title 24 Outdoor Lighting Controls
- Commercial Lighting Controls: A Buyer's Guide
- Commercial Occupancy Sensor Buying Guide
- Wired vs Wireless Commercial Lighting Controls
- What Is 0-10V Dimming?
Frequently asked questions
What is the difference between a daylight sensor and a photocell?
A photocell, properly a photocontrol, is a switch. It senses outdoor ambient light and turns exterior lighting on at dusk and off at dawn, with no dimming and no feedback. A daylight sensor, properly a photosensor, measures the light level inside a space and continuously dims the electric lighting as daylight comes through windows or skylights. Photocontrols are an exterior scheduling device; photosensors are an interior energy-code device that requires dimmable drivers and commissioning.
What is the difference between an open-loop and a closed-loop photosensor?
A closed-loop sensor points into the room and reads the combined daylight and electric light, so the lighting it controls feeds back into its own measurement. That makes it more accurate and suits a single zone where the delivered light level matters. An open-loop sensor is aimed at the daylight source and deliberately excludes the controlled fixtures from its view, so one reading of available daylight can drive many zones. Open loop cannot tell that blinds have been closed; closed loop can, because it is reading the room.
Where should a daylight sensor be mounted?
Follow the manufacturer's instruction for the specific device. Lutron's submittal for its Radio Powr Savr wireless daylight sensor directs installers to point the sensor's arrow at the nearest window at a distance from the window of one to two times the effective window height. In general, keep direct sun off the sensor, do not aim a closed-loop sensor at a bright desk or floor that will dominate its reading, and keep the controlled fixtures out of an open-loop sensor's field of view.
Does a daylight sensor need continuous dimming, or is switching enough?
Both are permitted. California's Title 24 does not require continuous dimming: Section 130.1(d) requires the control to adjust lighting via continuous dimming or the number of control steps provided by the multilevel controls, so stepped daylighting control is code-legal. The practical argument for continuous dimming is behavioral rather than regulatory. A visible step change is what prompts occupants to override the system, whereas a smooth ramp is generally not perceived. Specify continuous dimming where the budget allows, but a stepped system is not by itself a compliance problem.
When does the energy code require daylight-responsive controls?
In California, Title 24 Part 6 requires them once the general lighting in a daylit zone crosses a wattage threshold. Under the 2025 Energy Code, effective 1 January 2026, that threshold is 75 watts of general lighting in any primary sidelit, secondary sidelit or skylit daylit zone, reduced from 120 watts under the 2022 code. Outside California, the governing document is whichever the jurisdiction has adopted, an edition of the IECC or ASHRAE 90.1, and the thresholds and exemptions differ by document and by edition. Confirm the governing edition and any local amendments with the authority having jurisdiction.
Do wireless daylight sensors need batteries changed often?
Not on a normal maintenance cycle. Lutron rates its Radio Powr Savr wireless daylight sensor for a 10-year battery life on a single CR 2450 lithium cell. The more common wireless constraint is radio range rather than battery life: for the Radio Powr Savr models covered by that submittal, Lutron specifies that local load controls sit within 60 feet line of sight, or 30 feet through walls, of the sensor. Ranges vary by model within the family, so check the figure for the exact part number.
This guide is general information. Code thresholds, product specifications and manufacturer compliance statements are sourced from the California Energy Code, Energy Code Ace, ETC, Lutron and Legrand/Wattstopper published materials and may change without notice. Confirm current figures and the governing code edition with the authority having jurisdiction and the manufacturer spec sheet before final purchase.