PAR Meter - Grow Light PPFD and DLI Meter
Measure the light your plants actually use — PPFD and DLI over the whole day. Pick your plant type and see whether the light is too weak, right or too strong for your plant.
Is your plant getting enough light? Do not guess. The PAR Meter app lets you know. It measures the light your plants actually use: PPFD (Photosynthetic Photon Flux Density), and DLI (Daily Light Integral) over the whole day. Choose your plant type and the gauges show a the recommended range, together with advice on what to do to optimize the light for your your plats. Whether you grow houseplants on a windowsill, herbs under an LED panel, or tomatoes in a grow tent, this app lets you hang your lamp at the right height, find the dark corners, and compare one grow light with another.
Accuracy Above All
For the best possible accuracy, this app uses the built-in light sensor on Android phones and the light reflected off a white paper on iPhones. Both these methods eliminate the need for an external hardware device, while keeping the accuracy the best possible.
While seldom needed, there is also an advanced calibration feature that can be used for increased accuracy on your specific device.
No Diffuser Needed
You may have read that you need a diffuser in front of the camera to measure PPFD with a phone. That is true if the app looks at the lamp with the camera. This app does not.
Android: The Light Sensor Is Made for This
The app uses the phone's built-in ambient light sensor, which measures light from all directions, just like a handheld light meter. Pre-calibrated, it gives good readings right away on the vast majority of phones.
iPhone: Paper Reflection
The camera measures the light reflected off a sheet of matte white paper, which collects light from all directions. How much light passes through a diffuser depends on its material and thickness, so a homemade diffuser has to be calibrated first. White printer paper reflects light in a much more predictable way, so the app gives good readings right away.
No hardware to buy, nothing to clip on, and no compromise on accuracy.
We have measured light with smartphones for more than 10 years, so you can trust our meters.
Why Measure Grow Light?
Plants Do Not Grow on Lux
Plants grow on PAR — Photosynthetically Active Radiation, the light between 400 and 700 nm that photosynthesis can actually use. The human eye is most sensitive to green light, while plants use red and blue light the most. A lamp can look bright to the eye and still deliver very little usable light, or look modest and deliver plenty. That is why growers measure PPFD rather than lux or lumens.
Too Little or Too Much Light
Too little light gives slow, stretched, pale growth and leggy seedlings. Too much light wastes electricity and can bleach or burn leaves. Somewhere in between is the range where your plant thrives — and that range is different for a pothos, an orchid, a basil plant and a tomato. Measuring tells you where you are.
The Right Height, the Right Hours
Light falls off quickly with distance, and most lamps are much brighter in the middle than at the edges. With a meter you can set the lamp height above the canopy, map the whole grow area to find its dark corners, and work out how many hours per day the light should be on to reach the daily light your plant needs.
Usages
Houseplants and Windowsills
Find out whether the spot you picked for your monstera, pothos or orchid actually gets enough light, and how much a small grow lamp would add. Compare a north window with a south window, or the shelf by the window with the one across the room.
Grow Tents and Grow Rooms
Set the height of your LED grow light, quantum board or HPS lamp above the canopy, and map the PPFD across the whole tent to see how even the light really is. Check that seedlings, vegetative plants and flowering plants each get the level they need.
Herbs, Leafy Greens and Microgreens
Basil, lettuce, spinach and microgreens under a kitchen LED panel or a T5 fluorescent: measure the DLI and see whether the light period should be longer or shorter, or the lamp closer.
Greenhouses, Hydroponics and Vertical Farming
Measure daylight in the greenhouse through the seasons, and check the supplemental lighting in hydroponic, aquaponic and vertical farming setups. Save measurements with comments to document the light map of a whole grow space.
Aquariums, Terrariums and Vivariums
Planted aquariums, terrariums and vivariums need the right light too. Measure at the top of the plants to see whether the lamp delivers what the plants require.
Comparing Grow Lights
Compare LED grow lights, quantum boards, COB lamps, fluorescent tubes and HPS lamps at the same distance, and see what a lamp actually delivers in your setup rather than in the manufacturer's test chamber.
Education
A practical tool for teaching about PAR, PPFD, DLI and photosynthesis in horticulture, botany and agriculture courses, and for school and hobby projects on plant growth.
Understanding PAR, PPFD and DLI
What Is PAR?
PAR stands for Photosynthetically Active Radiation — the light between 400 and 700 nm that plants use for photosynthesis. It is a range of wavelengths, not a quantity. What you measure is how many photons within that range arrive at the plant, which is PPFD, and how many arrive over a day, which is DLI.
What Is PPFD?
PPFD, Photosynthetic Photon Flux Density, is measured in µmol/m²/s — micromoles of photons per square meter per second. As a rough guide: low-light houseplants are happy around 50-150, herbs and leafy greens want roughly 150-400, plants in their vegetative stage do well at 400-600, and flowering or fruiting plants typically want 600-1000. Full summer sunlight outdoors is about 2000.
What Is DLI?
DLI, Daily Light Integral, is measured in mol/m²/day and is simply the PPFD multiplied by the number of hours the light is on: DLI = PPFD × hours × 0.0036. As a rough guide: low-light houseplants get by on 2-5, herbs and leafy greens want roughly 12-20, and most fruiting or flowering crops want 20-60. A full summer day outdoors gives around 60. Note that the DLI shown is an estimate: it assumes that the current light level stays the same for all the selected hours, as the app does not add up the light received throughout the day. Daylight changes with the time of day, clouds and shade, so check the light at different times of day before deciding to move a plant.
How Accurate Are the Measurements?
Accuracy depends above all on selecting the correct light source type, and then on proper measurement technique. For daylight and discharge lamps such as HPS and metal halide the spectra are well known and the estimate is normally good. LED grow lights vary much more between models, so if the manufacturer publishes a µmol per 1000 lx figure for your fixture, use that. The app is most useful for practical field estimates and comparisons — between spots, heights and lamps. It is not a replacement for a calibrated quantum sensor when laboratory-grade accuracy is required.
Key Features
🌱 PPFD
Photosynthetic Photon Flux Density in µmol/m²/s — the amount of usable light arriving at your plants right now. This is the number growers use when they talk about how strong a light is at the canopy. The main gauge is logarithmic, so you get the same reading precision on a dim windowsill as under a strong lamp.
📅 DLI
Daily Light Integral in mol/m²/day — the total amount of usable light your plants receive over a whole day, which is what actually drives growth. Set your light period per day (the photoperiod) and the app calculates the DLI from the measured PPFD.
🪴 Plant Types With Recommended Light Levels
Choose among houseplants (low light, foliage and flowering), orchids, succulents and cacti, seedlings, cuttings, microgreens, leafy greens, herbs, root vegetables, fruits and berries, fruiting vegetables, and high-light crops in the vegetative and flowering stages. Each plant type has a recommended DLI range, which is shown as a green, yellow and red band on both gauges.
👍 Clear Guidance
Under the gauges, the app tells you in words whether the light is far too weak, a bit weak, right, a bit strong or far too strong for the selected plant — and suggests what to do about it: move the lamp closer or further away, dim it, change the number of hours, or move the plant.
💡 Light Source Types
The conversion from lux to PPFD depends entirely on the spectrum of the lamp, so selecting the correct light source type is the single most important setting in the app. Choose among daylight, white LED, LED grow lights, fluorescent, CFL, HPS, MH, CMH, halogen and incandescent, or enter the µmol per 1000 lx figure published by the manufacturer of your lamp. See "Choosing the Right Light Source Type" below.
☀️ Illuminance in Lux and Foot-Candles
Illuminance is always shown alongside PPFD and DLI — so the app also works as an ordinary lux meter.
⏸️ Hold
Freeze the reading with the Hold button before you take the phone out from among the plants to read it off. On Android you can also use the volume up button to freeze the reading, and the volume down button to save the measurement.
💾 Save Measurements
Store measurements with timestamps and comments. Each saved measurement keeps its illuminance reading, so you can change the light source type, the light period or the plant type afterwards and the PPFD, DLI and recommendation are recalculated. Export everything to a CSV file to document the light map of a grow space.
🔧 Built-In Calibration
While pre-calibrated for good results out of the box, the app offers optional calibration against a reference lux meter, or against a PAR meter or quantum sensor, for maximum accuracy on your specific device. Calibration adjusts the lux value, and thereby both PPFD and DLI. Use it with care: an incorrect calibration can make the readings completely wrong, and calibration does not make up for selecting the wrong light source type.
How to Measure Grow Light
On Android: the Light Sensor
Hold the phone where the leaves are, with the light sensor facing the light. The sensor is usually located just above the screen. Do not shade it with your hand, and make sure that no case or screen protector covers it. Some light sensors update slowly in dim light, so wait a second for the value to settle before you hold or save the reading. On the few Android phones without a light sensor, the app uses the camera and a white paper instead, just as on iOS.
On iOS: a Sheet of White Paper
Hold a plain, matte sheet of white paper where you want to measure, and point the camera at the paper, not at the lamp. Fill the whole measurement area with the paper and avoid shadows from your hand and phone. Measuring the light reflected from a white paper makes the reading much less sensitive to the angle than a traditional incident measurement, which usually suits plants well, as their leaves face in many different directions.
Measure at Several Points
Light falls off quickly with distance, and most lamps are much brighter in the middle than at the edges. Measure at the height of the leaves, at several points across the grow area and not just under the center of the lamp, to find out how even your light is.
Only the Light You Want to Measure
If you want to measure one lamp on its own, make sure that no other lamp and no daylight reaches the measurement spot. Let the lamp warm up first: HPS and metal halide lamps need 5 to 15 minutes, and even some LEDs change slightly during the first minutes.
Choosing the Right Light Source Type
The app measures illuminance and converts it to PPFD with a factor that depends on the spectrum of the lamp. Selecting the correct light source type is therefore the most important thing you can do for accurate readings.
Daylight
Natural light from the sun and sky, for outdoor, greenhouse and windowsill measurements.
White LED
Ordinary white LED bulbs and panels that are not sold as grow lights.
LED Full Spectrum
Grow lights with white or warm white diodes only, such as most quantum boards.
LED Full Spectrum + Red
White grow lights with extra deep red (660 nm) diodes, often visible as a few red dots among the white ones.
LED Red/Blue
Grow lights that glow purple, made of red and blue diodes. These are hard to measure accurately with a smartphone, so if you know your lamp's µmol per 1000 lx figure, use that instead.
Fluorescent and CFL
Fluorescent tubes such as T5 and T8, including most propagation lights, and compact fluorescent lamps, the energy saving bulbs with a folded or spiral tube.
HPS
High Pressure Sodium lamps with their orange light, in single-ended or double-ended versions.
MH and CMH
Metal Halide lamps, a bluish white HID lamp often used for the vegetative stage, and Ceramic Metal Halide lamps, also sold as LEC, in 4200 K (whiter) and 3100 K (warmer, often called agro or grow) versions.
Halogen and Incandescent
Halogen bulbs and spotlights, and ordinary old style light bulbs with a glowing filament.
µmol per 1000 lx
Many manufacturers of grow lights publish how many µmol/m²/s their lamp gives per 1000 lux. If yours does, select the matching entry, such as "15 µmol per 1000 lx", for the best accuracy.
Frequently Asked Questions
Do I need a diffuser to measure PPFD with a phone?
Simply pointing a camera upwards at a lamp does not work well, and that is why a diffuser is often recommended. But on Android the app does not use the camera: it uses the phone's ambient light sensor, which is made to measure the light falling on the phone just like a physical light meter, so no diffuser is needed. On iOS, the paper reflection does a better job than a diffuser, see iPhone: Paper Reflection.
Why does my reading differ from the lamp's specification?
Manufacturer PPFD figures are normally measured directly under the center of the lamp at a stated distance, in a reflective test chamber, with a new lamp. Your grow space is rarely any of those things. Distance, angle, reflection from the walls and the age of the lamp all change the light that actually reaches the leaves.
Does the app work with purple red/blue grow lights?
Yes, but with lower accuracy. The light from red/blue LEDs looks dim to the human eye, and therefore to any lux based method, while plants use it well. For these lamps, use the µmol per 1000 lx figure from the manufacturer if there is one.
Does the app measure the DLI over a whole day?
No, the DLI is calculated from the current PPFD and the light duration you select, assuming that the light stays the same all the time. This works well under grow lights, but daylight varies, so measure at different times of the day for plants on a windowsill or in a greenhouse.
I reinstalled the app, how do I get full access again?
Go to the Settings tab, select "Get Full Access," then tap the "Restore Purchase" link.
What Users are Saying in Official Reviews
"This app is perfect for growers. It's easy to use and very precise. I compared it to other measuring devices (not apps) and found only minimal deviations – making this app my top choice. I've since upgraded to the premium version and now only use this app because of its accuracy. I highly recommend it, but try it out for yourself 😉🫶"
Need Help or Have Ideas?
We're committed to making this app the best it can be. Your feedback matters to us, and we personally respond to every user who contacts us. Whether you have questions, need support, or have ideas for new features, please reach out to us at [email protected]
Wide Language Support
The app has full support for 40 different languages, making measurement accessible worldwide.
10+ Years of Measuring Light
More than ten years ago we released the world's first smartphone app for measuring the color temperature of light. Since then we have specialized in one thing: turning regular smartphones into accurate light and color meters. With more than one million downloads, our apps are used every day by photographers, growers, lighting designers, teachers and the curious, in more than 150 countries worldwide.
Download the PAR Meter app now
All the basic functionality of the app is free. To use the more advanced features a subscription or a one time fee is required.