Light Intensity Measurement: How Many Lux Does a Workplace Need?

The desk by the window feels bright all day, while two rows back people switch on their desk lamps by mid-afternoon. A light intensity measurement tells you whether that is just a feeling or a real shortfall. The short answer to the headline question: for writing, reading and data processing, both the European standard EN 12464-1 and the German workplace rule ASR A3.4 set 500 lux on the work surface. The area immediately around the desk needs at least 300 lux.

How much your number means depends less on the meter than on where, when and how you take it. A reading at the window at noon says little about the desk against the inside wall at 4 p.m.

What does a light sensor measure?

A light sensor for workplaces measures illuminance: how much light lands on a surface. The unit is lux (lx). Inside there is usually a photodiode whose spectral sensitivity is filtered to match the human eye. Without that correction, the sensor would count infrared light you never perceive as brightness, and readings would drift high or low depending on the light source.

On good sensors, a white diffuser dome over the diode weights light arriving at an angle correctly, which matters at a desk lit from the side by windows and from above by the ceiling.

There is an important difference between real measurement and simple bright/dark detection. A daylight switch only knows whether it is brighter or darker than one threshold. A measuring sensor returns a number in lux that you can compare with recommended minimums and log over weeks. The Milesight EM500-LGT belongs to the second group: according to its datasheet it measures from 0 to 100,000 lux with 1 lux resolution and ±3% accuracy. The light probe is a separate aluminum housing on a 3-meter cable, so the transmitter with its battery and LoRaWAN radio can hang on the wall while the probe sits where the work happens.

Keep in mind that 1 lux resolution only describes the steps in which the value is displayed. How closely it matches reality depends on accuracy, orientation and position.

How do you measure lux?

ASR A3.4 defines reference heights: for mostly seated work the horizontal reference plane is 0.75 meters above the floor, for standing work 0.85 meters. For a normal desk, that is almost exactly the desktop. If you know the height of the actual visual task, you can measure right there instead.

Lay the probe flat, sensing surface facing up. A probe tilted by just a few degrees “looks” more toward or away from the window, and the value shifts. The most common mistake when you measure lux, though, is your own shadow. Lean over the meter to read the display and you are measuring yourself. Stretch out your arm or read the value remotely on a screen.

One point is not enough for an assessment. The German rule calls for measurement points spread as evenly as possible across the reference surface, in other words a grid over the workstation area. You average the individual readings. On top of that, no point within the workstation area may drop below 0.6 times the average illuminance, and the lowest value must not fall where the main visual task takes place.

If you only want to check the electric lighting, daylight gets in the way. ASR A3.4 recommends measuring after dark. If that is not possible, take two readings right after each other, one with the lights on and one with them off, ideally under an overcast sky. The difference is the share contributed by the fixtures. Fluorescent and other discharge lamps should have at least 100 operating hours behind them before you measure.

Lux vs lumens: what’s the difference?

When people talk about light intensity at a desk, they almost always mean illuminance in lux. Lumens describe luminous flux, the total amount of light a lamp emits, no matter where it goes. One lux equals one lumen per square meter.

A simplified example shows the difference. If 1,000 lumens spread evenly over one square meter, you get 1,000 lux. Spread the same light over four square meters and you are left with 250 lux. The lamp has not gotten any dimmer; only the area has grown. Walls, ceilings and furniture also absorb or reflect light.

Distance has a strong effect, too. For a source that is close to a point, illuminance falls with the square of the distance. Double the distance and you get a quarter of the light. That is why a task lamp right above the work surface often helps more than a more powerful ceiling fixture.

Datasheets also list luminous intensity in candela, the light sent in one direction, and luminance in candela per square meter, how bright a surface such as a monitor looks. For whether your workplace has enough light, what counts is illuminance on the work surface in lux.

How many lux does an office desk need?

In Germany, ASR A3.4 “Lighting and visual contact” lists minimum values for tasks and areas in its Annex 3. For typical office work, meaning writing, reading and data processing, the figure is 500 lux. The European standard EN 12464-1 sets the same 500 lux for these tasks. This selection from the German rule shows how far the values spread:

The table makes it clear why one target for the whole building does not work. A corridor at 50 lux is fine; a drafting table at 500 lux is not. For the office desk, 500 lux remains the key number. Around the workstation, the rule asks for an average of at least 300 lux in the surrounding area whenever the workstation itself needs 500 lux or more, so your eyes do not keep adapting between bright and dim every time you look up.

The word “minimum” matters. The 500 lux have to be reached in everyday operation, not just on the day the lighting was installed. Dirty fixtures and aging lamps deliver less light over time, and the rule explicitly names this decline as a defect that triggers maintenance. It also notes that declining eyesight, for example with age, can call for higher illuminance. So 500 lux is a floor, not a comfort target.

Where should you measure light levels at a workplace?

ASR A3.4 separates the workstation area, meaning the desktop and the space directly in front of it, from the surrounding area next to it. The 500 lux apply to the desktop, specifically where you read and write. The middle of the desk in front of the keyboard is a good starting point. The windowsill is not, even if that is the most convenient place to put the probe.

Typical misplacements give you neat but wrong numbers. A probe under a shelf measures shadow; one on top of the monitor measures light that never reaches your paperwork. A sensing surface mounted vertically on the wall sees a different mix of window and ceiling light than the horizontal desktop.

In a room with several desks, start with a survey using a handheld meter at every workstation. Afterward you know where the darkest desk is, usually against the inside wall, behind a cabinet or behind a partition. A permanently installed light sensor belongs at that critical desk. If light levels there stay above the minimum, the rest of the room is usually covered too. A good reading at the window desk, on the other hand, proves nothing about the back row.

The Milesight EM500-LGT is built for exactly this setup: the transmitter mounts on a wall, pole or DIN rail, and the probe lies flat on the desktop, connected by its cable. Route the cable so nobody shifts or tilts the probe while tidying up. Note the position and orientation so later readings stay comparable, and keep the white diffuser clean, because dust drags every value down.

Daylight vs artificial light: how does illuminance change during the day?

Outdoor light levels vary enormously. On an overcast day you get roughly a few thousand up to about ten thousand lux; in direct summer sun, up to around 100,000 lux. Only a small share of that reaches an office desk, and the share drops noticeably with every meter away from the window. The German rule works with the daylight factor here: the ratio between indoor and outdoor illuminance under an overcast sky. A value above 2% at the workstation counts as sufficient.

A quick calculation shows why that rarely covers the whole need. Two percent of 10,000 lux outdoors gives 200 lux on the desk, so electric lighting has to supply the rest up to the 500 lux for office work. In winter, early in the morning and late in the afternoon, the daylight share drops even further. At midday, on the other hand, sunshine can push a desk near the window to several thousand lux. That is bright enough for glare and screen reflections to become the real problem, even though the minimum is far exceeded.

This is where a permanently installed sensor shows its strength. A daily curve over several weeks shows when the back desk drops below 500 lux, how much lowered blinds pull the value down and whether enough light arrives once the ceiling lights come on.

When something stands out, note the weather, blind position and which fixtures were on; a jump from 300 to 600 lux explains little if several things changed at once. Observed patterns lead to concrete fixes: a repositioned desk, a task lamp or different shading.

Illuminance sensor vs lux meter: which do you need?

A lux meter is a handheld instrument for snapshots: you measure point by point and get a picture of the lighting at that moment. For indicative measurements in the workplace, ASR A3.4 requires illuminance meters that meet at least class C under DIN 5035 Part 6. Smartphone apps do not. They use the phone’s ambient light sensor or camera, which are designed for display brightness, not calibrated for measurement.

A permanently installed illuminance sensor answers a different question: how the lighting at one specific desk behaves across days and seasons. Here is what each tool is good for:

The two instruments complement each other. For a documented check as part of a workplace risk assessment, you need a meter of the required class. The EM500-LGT datasheet does state ±3% accuracy, but no classification under DIN 5035-6. What the wireless sensor gives you instead is a clear view of when and where a follow-up check makes sense. After installation, compare the sensor once with a handheld meter at the same spot. If the reading is consistently off, the EM500-LGT lets you store a calibration value in lux as a correction.

How often should a light sensor report?

On the EM500-LGT, two intervals are set separately. The collection interval defines how often the sensor measures, every 60 seconds by default. The reporting interval defines how often it sends a value over LoRaWAN, every 10 minutes by default, adjustable from 1 minute to 18 hours. You configure it with a smartphone app over NFC, on a PC over USB-C, or by downlink over the network. To receive the data, you need a LoRaWAN gateway within range.

For most workplaces, 10 minutes is a good compromise. A daily curve with six points per hour reliably shows when the lights come on, what the blinds do and how daylight fades in the afternoon. A cloud that covers the sun for two minutes can slip between two reports, but that does not matter for the 500 lux question. If you want to pin down glare or reflection periods at a window desk, shorter intervals of 2 to 5 minutes help. For long-term tracking of whether the installation degrades over months, 30 to 60 minutes is enough.

Every extra report costs battery and airtime. Milesight states up to 10 years of life for the 19,000 mAh battery, measured at a 10-minute interval and 25 °C under lab conditions. Halve the interval and battery life drops noticeably. If the gateway goes offline, the sensor can store up to 1,000 readings locally and resend them later, provided you enable data storage and retransmission. There is also a threshold alarm: the sensor then reports immediately, outside the regular interval, when illuminance crosses a set limit. Always show the timestamp of the latest value on your dashboard.

Where can you buy a light sensor?

Recap the criteria first: the sensor should deliver real lux values, not just bright or dark. Its range needs to cover everything from a dim corridor to direct sun at the window, so up to around 100,000 lux. Look for an accuracy figure in the datasheet, not just the resolution. A separate probe on a cable is a big advantage, because you can place it flat at desk height while the transmitter and battery stay on the wall. Check whether the measurement and reporting intervals are adjustable and how long the battery lasts at the interval you want. The radio standard and gateway have to fit your existing infrastructure. And for a formal check under ASR A3.4, you also need a class C handheld lux meter: the permanent sensor shows you the trend, the handheld instrument provides the documented proof.

If you want continuous light intensity measurement that logs your desks over weeks, you can find suitable models in our light sensors category.