How to measure and test the light in your rooms

A room survey, readings with a meter or a phone, and tests for glare, shadows, flicker and color you can run without any instrument, recorded so later changes can be judged against them.

1. The room survey and fixture inventory Understand

The other chapters each change something in a room and ask whether it helped. That question has an answer only if you wrote down what the room did before. A room survey is that record: one row per room, describing what the room is for and what is wrong with its light now. A fixture inventory is the matching list of every light in the room and what is in it. Together they are the "before" that the rest of this manual compares against.

What to record for each room

The rooms sheet has a column for each of these. Fill them in from where you usually sit or stand, at the time you usually use the room.

• What happens there. Reading, cooking, homework, video calls, sewing, watching television. Each activity has its own surface: the page, the counter, the keyboard, the face in the mirror. Those surfaces are where you will measure.
• When it is used. A living room used mostly after dark is a different problem from one used mostly at noon. Test at the times the room is actually used.
• Which way the windows face. How light works in a room explains why window orientation changes the daylight you already have.
• Walls and ceiling: light, medium or dark. How light works in a room explains surface reflectance. The lumen calculator in Layers, fixtures and glare uses this answer, so a guess now is better than a blank.
• Problems, in your own words. "Can't read on the sofa," "the kitchen ceiling light glares off the counter," "bulbs don't match." Use the words you would use to a friend; the tests below turn them into something you can check.

What to record for each fixture

The fixtures sheet takes one row per lamp or fixture: where it is, the bulb type and base, lumens, kelvin and CRI where the package or base says them, whether the bulb is marked dimmable, the maximum wattage printed on the fixture, whether the fixture is enclosed, and what switch or control runs it. Bulbs, labels and color shows how to read each of those on a bulb label. Leave a cell empty rather than guess, and note where the answer would come from ("CRI not printed; check the maker's sheet").

Turn a light off and let the bulb cool before you unscrew it to read the base. Two numbers matter for the safety walk later: the maximum wattage on the fixture and the actual watts of the bulb in it.

Confirmed, provisional, unresolved

Every finding in the sheets carries a status. Confirmed means you checked it directly and would get the same answer again: the fixture label says 60 W maximum. Provisional means you have a result but it rests on something uncertain: a phone reading, a single evening, a flicker video that showed nothing. Unresolved means you do not know yet and you have written down who or what will settle it. Most of what this chapter produces is provisional. That is the honest status for a phone reading, and it is still useful, because the same phone in the same spot on another evening tells you whether a change helped.

2. Measuring light: meters and phones

A light reading is a measurement of illuminance: how much light arrives on a surface, in lux or footcandles (How light works in a room explains lumens and lux; 1 footcandle is about 10.76 lux). The instrument for it is an illuminance meter, which most people call a light meter. Readings depend on where the sensor is, which way it faces and what is shading it, so the method matters as much as the instrument.

How to take a reading

Two government procedures written for workplace inspectors agree on the basics. Canada's Labour Program says to lay the meter on the surface being evaluated with the sensor facing up, and for readings across a room to hold it level with the sensor up (Employment and Social Development Canada, Measurement of Lighting Levels in the Work Place). It warns that shading the sensor gives an artificially low reading, and suggests the meter's hold button so your head does not affect the number. It takes four readings at representative points and averages them, rather than picking the darkest or brightest spot. The Canadian Centre for Occupational Health and Safety (CCOHS) also takes several readings and averages them, with people in their normal positions (CCOHS, Lighting Ergonomics: Survey and Solutions). Convention

At home, that becomes a short routine:

1. Choose the spot. The surface where the activity happens: the arm of the sofa where the book rests, the counter where you chop, the desk where the paper lies. Mark it with a piece of tape so the after reading lands in the same place.
2. Set the room as you use it. The lights you normally have on, curtains as they usually are, at the time of day you use the room. For a first survey, after dark removes daylight from the comparison.
3. Lay the meter or phone flat, sensor up. Step back or lean away so your head and body are out of the light. On a phone, the sensor or camera is on the face you are reading, so lay the phone flat on its back and read the screen from the side, or use the app's hold or delay function.
4. Take three readings, lifting and replacing the device each time, and average them. Three is our shortcut for one spot; the Canadian procedure's four points are for an area. Write the average and the device in the tests sheet.

The workplace procedures measure at a fixed height, 1 m above the floor in the Canadian procedure and 76 cm (30 in) in the CCOHS sheet. For a home, the surface you use is the more useful plane: the number you care about is the one on the page. Convention

What a real meter specifies

Two specifications separate an instrument from a gadget. Konica Minolta's brochure for its laboratory-grade T-10 meter explains both (Konica Minolta, T-10 and T-10M Illuminance Meters). The first is spectral match: the sensor should respond to colors the way the eye does, following the CIE eye-response curve; the mismatch is quoted as f1′, within 6 percent for that meter. The second is cosine correction: light arriving at a slant lands on a flat surface more weakly than light from straight above, in proportion to the cosine of the angle, and the sensor has to follow that rule; the error is quoted as f2, within 3 percent there. Product-specific Many inexpensive meters do not publish either figure. If yours does not, treat it the way you would treat a phone: consistent with itself, not proven against anything else. The Canadian inspectors allow for 10 percent error even on a calibrated professional meter (5 percent for the instrument, 5 more for calibration), which shows how far "a reading" is from "the light level."

Phones as light meters

A free light-meter app turns a phone into something that reports lux. How well it does that has been tested, and the answer is: it depends on the phone, and it is worst at home light levels.

• A 2022 University of Maribor study compared ten phones with a reference meter under an LED panel (Vujica Herzog and colleagues, Comparison between Lux Meter Apps and Illumination Measuring Devices, DAAAM 2022). At 400 lux one Samsung phone read within 1 percent; one Huawei phone was off by 99 percent. Android phones read a dedicated light sensor and the app made almost no difference; iPhones have no exposed light sensor and read through the camera, so on an iPhone the result depends on the app. Accuracy did not follow price, age or brand. The authors concluded that most phones give meaningful data for home use, and that precise work needs a calibrated meter.
• A 2020 study in Citizen Science: Theory and Practice calibrated three phones against a meter. Even after calibration, the remaining error grew as light fell: about 12 percent in bright light, 16 percent in the middle range and 18 percent in the lowest range (Odenwald, Smartphone Sensors for Citizen Science Applications). Home evening light sits in the worse ranges. The author notes that phones lack a meter's diffusing dome and that a hand or head easily shades the sensor.
• A 2018 poster from what is now Toronto Metropolitan University tested 21 phone models with 70 participants and found readings inconsistent even between phones of the same model (Messalam and Abdoli-Eramaki, 2018). Android apps were much closer to the meter than iOS apps.

So a phone reading is not a measurement of the light level. Test it What a phone does well is compare: the same phone, the same app, the same spot, the same time of evening, before and after a change. If the phone reads 38 lux before and 81 after, you can say the phone read about twice as much, and that is a real finding about your change even if neither number is the true light level. Every reading in this manual is used that way.

3. Published targets, and what they were written for

Once you have a reading, the natural question is what it should be. Several publishers give targets. They disagree by a factor of about ten, they measure in different places, and none of them was written for one person in one home. The table shows the ones we could read for free.

What each column is:

• The Defense Department's criteria (Unified Facilities Criteria 3-530-01, 2023, section 3-9 on housing) are the only free government document we found with room-by-room residential targets (UFC 3-530-01, Interior and Exterior Lighting Systems). They are averages for military family housing, mostly measured at the floor, and they say they are based on the IES Lighting Library. They are low because the floor gets less light than a table and because they set a maintained average for the room, not the light on a book. Convention
• The commonly cited ranges are the footcandle table that circulates on retail and university pages. We cite Penn State's energy course notes, which attribute it to the Illuminating Engineering Society without naming an edition (Penn State EGEE 102, How Much Lighting is Needed). The ranges match older IES category tables. We call them "commonly cited, attributed to older IES guidance," not an IES recommendation. Convention
• The Lighting Research Center's floors come from its former design guidance for older adults (archived copy of a page the Lighting Research Center no longer hosts): ambient light of at least 300 lux, at least 1,000 lux on tasks with fine detail or low contrast, and at least 50 percent more light than younger people find comfortable, with glare kept down. They apply to older eyes, which How light works in a room covers under the eye and age. Convention
• The current IES residential practice (RP-11) sets levels by age group, but it is sold, not published free, and we have not read it. We do not quote values from it.

Real homes measure low against all of these. In a small Pacific Northwest National Laboratory sample of 30 lighting professionals' homes, the median living room in the evening gave about 23 lux at the eye and the median kitchen about 104 (Miller and Kinzey, Home Nighttime Light Exposures, PNNL 2018). Those were readings at the eye, not on a surface, so they do not compare directly with the table; they show only that dim is normal.

4. Tests without a meter: glare, shadow, flicker and color

Most of what makes a room hard to use is not the amount of light. It is where the light comes from: a bright source in view, a reflection on the page, a shadow on the work, a light that flickers, a color that looks wrong. These tests need no instrument. They come mostly from the CCOHS survey sheet, written for offices, and they work the same way at home.

Glare: the book-block test

Direct glare comes from a bright source in your field of view. CCOHS describes a simple check. Sit or stand where you normally do. Look at something across the room at eye level. Then hold up a book or a piece of cardboard to block the light path from the fixture to your eyes, without blocking the object. If the object becomes easier to see, that fixture is producing glare. Test it Repeat for each light you can see from the seat. Write the result in the tests sheet's glare column: none, direct, reflected or both.

Reflected glare: the blocking test and the mirror test

Reflected glare is a bright source seen as a reflection in the thing you are looking at: a shine on a glossy magazine, a white streak on a laptop screen, a hot spot on a polished counter. CCOHS gives two checks. For the blocking test, look at the task from your normal position and block the light falling on it from in front or above; if details become easier to see, reflections are the problem. For the mirror test, lay a small mirror face up on the work surface; if you can see a fixture in it from your normal position, that fixture is the source of the reflection. Test it A screen at a desk is the usual place this shows up, and Kitchens, bathrooms and desks uses the same test on desks and screens.

Shadow on the work

Stand or sit where the work happens, with the lights you normally use, and look at the surface. Is your own head, hand or body shading it? A single ceiling fixture behind you puts your shadow on the counter, the sink or the page; the Lighting Research Center's kitchen guidance names exactly that problem (Layers, fixtures and glare covers where task light goes). Record none, some or heavy in the shadow column, and note which light is behind you. Test it

The slow-motion flicker check

Flicker is a rapid, repeated change in a light's output. Much of it is too fast to see directly. Bulbs, labels and color explains what flicker is and why dimmers make it worse. A phone's slow-motion video can sometimes catch it, because the camera samples the light faster than the eye notices it.

1. Set the phone's camera to slow-motion video.
2. Point it at the bulb or at a white surface the bulb lights, and record two to three seconds. For a light on a dimmer, record at full, middle and the lowest setting you use.
3. Play it back. A steady light stays the same brightness. Flicker shows as pulsing, flashing or dark bands rolling across the picture (banding).
4. Write "seen" or "none seen" in the flicker column, with the dimmer setting.

These steps follow Waveform Lighting's description of the method (Waveform Lighting, Diagnosing and Resolving LED Flicker Issues). Waveform sells bulbs, and it says plainly that this is a visual check and that measuring flicker takes equipment costing several hundred dollars. Preference

The check can flag flicker. It cannot show that a bulb is free of it. A camera's result depends on its frame rate and exposure, and a clean video proves only that this camera did not catch anything. The Pacific Northwest National Laboratory tested eight handheld flicker meters, including a free phone-camera app, against a laboratory reference (PNNL, Characterizing Photometric Flicker: Handheld Meters, 2018). The app's percent-flicker readings were off by about 11 points on average at full output and about 33 points when the lights were dimmed. PNNL called it a handy way to find that a problem may exist, to be followed up with better equipment. The IEEE flicker practice's committee chair makes the same point in a DOE-hosted summary: sometimes a digital camera picks up flicker (Miller and Lehman, Flicker: Understanding the New IEEE Recommended Practice). So "seen" is a useful result and "none seen" is provisional. Test it

If you see flicker at a setting, stop using the light at that setting until it is sorted out; Switches, dimmers and evening light covers bulbs, dimmers and compatibility. If light or flicker gives you headaches or eye pain, that is a question for a doctor. Lights that flicker or dim when an appliance starts are a different matter, covered in the safety walk below.

Color

Hold the same three things under each light in the room: a sheet of plain white paper, the back of your hand, and something saturated red (a tomato, a red book, a red pen). Look for a white that turns yellow, blue, green or pink between lights, skin that looks gray or ruddy, and a red that goes brown. Write what you see in the color note column, with the kelvin and CRI from the fixtures sheet beside it. Bulbs, labels and color explains color temperature and color rendering. DOE's color fact sheet is blunt about what numbers can and cannot do: there is no substitute for looking. Test it

Test at the times the room is used

A room can pass at noon and fail at 9 p.m. Do the first readings after dark, because that is when only your electric light is working and the comparison is cleanest. Then repeat the glare and color checks in daylight if the room is used then: a window behind a screen or in front of a reading chair is a glare source the evening test misses.

5. The safety walk

The safety walk is looking, not fixing. You walk the room with the safety checklist, touch plates with the back of a hand, read labels, and look at cords. What you change yourself is limited to bulbs and lamps: a bulb that is wrong for its fixture, a lamp that is too close to a curtain, a cord that runs under a rug. Anything warm, buzzing, discolored or tripping goes to a licensed electrician, and Working with an electrician covers what to tell them.

ESFI's line on warmth is useful: a plate that is warm to the touch may be okay, but hot is not. The Consumer Product Safety Commission's checklist adds that some dimmer switches become warm in normal use, and that unusually warm outlets or switches may mean unsafe wiring (CPSC, Home Electrical Safety Checklist). ESFI also suggests standing across the room and looking for teardrop-shaped darkening around and above plates.

The checklist, item by item

• Bulb within the fixture's maximum wattage. NFPA says to use bulbs that match the recommended wattage on the lamp or fixture and, for LED replacements, to read the labels to make sure the bulb suits the fixture and location. Compare the bulb's actual watts (the label's Energy Used line) with the fixture's maximum, not the "equivalent" watts on the front of the box. If a fixture has no wattage label, CPSC's checklist says to use a bulb of 60 watts or less, and 25-watt bulbs in unmarked ceiling fixtures that take miniature bulbs. Convention
• Bulb rated for an enclosed fixture where the fixture is enclosed. A glass dish or globe that closes around the bulb traps heat. Many LED packages say "not for use in totally enclosed luminaires"; Bulbs, labels and color covers heat and LED life. Product-specific
• Damp or wet marking where needed. Near a sink, tub or shower, Kitchens, bathrooms and desks explains the markings and leaves placement to the electrician and inspector.
• No cords under rugs, across doorways or pinched by furniture. NFPA lists all three, and says extension cords are for temporary use only.
• Lamps and bulbs away from anything that can burn. NFPA names furniture, bedding, curtains and clothing.
• Cover plates on every switch and outlet. NFPA says all receptacles and switches should have them.
• No switch or plate that is hot, discolored, buzzing or smells; no flicker or dimming when appliances start; no repeated tripping. Any yes here is the Caution above.
• Testing-laboratory mark on lamps and plug-in lights. CPSC's checklist names UL, ETL and CSA as examples. OSHA's program page explains that each recognized laboratory has its own registered mark and that the mark means the laboratory tested and certified the product to a safety standard (OSHA, Nationally Recognized Testing Laboratory Program). UL is one of several.
• Lamps and bulbs checked against the recall list. CPSC's recall search has categories for light bulbs and floor and table lamps (CPSC, Recalls and Product Safety Warnings). SaferProducts.gov lets you read and file reports about unsafe products; CPSC says it does not vouch for the accuracy of reports the public submits (SaferProducts.gov).

When you change a bulb as a result of the walk: light off, bulb cool, and the new bulb's actual watts within the fixture's label. Screw it in firmly; ESFI notes that a loose bulb may overheat.

6. See it: Marisol's first evening of testing See it

Free sources that show the methods in this chapter:

• CCOHS, Lighting Ergonomics: Survey and Solutions: the book-block, blocking and mirror tests in the original workplace wording, plus the two-reading reflectance method.
• University of Maribor, lux meter apps compared: the phone-by-phone tables behind the "1 to 99 percent" figure, if you want to see how your kind of phone did.
• PNNL, Characterizing Photometric Flicker: Handheld Meters: why a phone app's flicker reading is a flag, not a measurement.

7. Practice: one room, one evening Practice

Minimum: the rooms sheet row for your first room, three readings at one spot averaged, the book-block test, the flicker check on any dimmed or unsteady bulb, and the safety checklist for that room. Time: about 45 minutes after dark; 15 more for the daylight checks. Materials: a phone with a free light-meter app or a light meter, painter's tape, a book or piece of cardboard, a small mirror, a sheet of white paper, something red.

1. Fill in the rooms row. Open the rooms sheet. Write what happens in the room, when, the windows, light, medium or dark for walls and ceiling, and the problems in your own words.
2. Tape the spots. One spot per activity: the sofa arm, the table, the desk, the counter. Three spots is plenty for a first room.
3. Read each spot three times after dark. Lights as you normally use them, device flat, sensor up, your body out of the way. Average the three and write the average, date, time, device and "before" in the tests sheet. Mark it provisional if it came from a phone.
4. Run the book-block test from the main seat. Record none, direct, reflected or both, and which light.
5. Check for shadow on the work. Record none, some or heavy, and which light is behind you.
6. Record slow-motion video of each dimmed or unsteady bulb. Full, middle and low on a dimmer. Record seen or none seen.
7. Walk the safety checklist. Warning signs first. Tick each item and note anything you changed (a bulb, a lamp moved, a cord rerouted).

8. Your rooms Your rooms

9. Check your work Check your work

Criteria

• Does the tests sheet have a dated row for your first room with the time, the device, an average of at least three readings, and "before"?
• Is each spot marked so the after reading can land in the same place?
• Does the row carry a glare result, a shadow result and a flicker result, each with the light it refers to?
• Does every phone reading say provisional?
• Is the safety checklist complete for the room, with any warning sign sent to an electrician rather than handled yourself?
• Have you avoided grading the room against a single target number?

Worked example

Marisol's row (fictional) meets each criterion: dated 2026-10-06, 8:30 p.m., phone app, three averaged readings at three taped spots, direct glare from the ceiling dish, heavy shadow at the desk, flicker "none seen," all marked provisional. Her safety checklist is complete, with one open item (the enclosed-fixture marking on the bulbs) and the source that will settle it. She did not write "too dark" as a finding; she wrote the readings and the problems, and left the verdict to the before-and-after in Your lighting plan.

Common mistakes

• Reading with the phone in your hand, so your head shades the sensor. The number drops, and the after reading no longer compares with it.
• Switching phones or apps between before and after. On an iPhone the app changes the result; on any phone, a different phone does.
• Testing at noon in a room used at night.
• Treating "none seen" on the flicker video as proof that a bulb does not flicker.
• Comparing a sofa-arm reading with a target measured at the floor.
• Opening a switch plate to look at a warm switch. Leave it closed and call an electrician.

What is still unresolved

This chapter cannot say how much light your room should have, or whether your phone reads true. It gives you a dated "before" and a list of problems with the light that causes each. Layers, fixtures and glare turns those into a plan with a lumen range for each layer, and Your lighting plan retests the same spots with the same device to see whether the plan helped.

10. Knowledge check and scenario

Checklist

• The tests sheet has a dated row for my first room with an average reading, a glare result and a flicker result.
• The safety checklist for my first room is complete, warning signs first.
• The rooms sheet has a row for my first room, and the reading spots are marked.
• The fixtures sheet lists every light in the first room, and I have run the color check. (Go deeper; optional)
• I have taken a four-point line or calibrated my phone against a meter. (Go deeper; optional)