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Being copied is a compliment! Photone was the first app ever to measure PAR on a phone. Today, several apps show a PPFD value and claim to measure PAR. The fact that we're lumped together with copycat apps is an issue we'd like to address … using numbers rather than adjectives.
Same unit, different instruments
If you put two phones under the same lamp, one running Photone and one running another PPFD app, both show a number in µmol/m²/s. Same unit, same look, and both readings react when you move the lamp. From the outside, they look like the same kind of tool.
However, they are not! Let's explore the difference.
How a phone arrives at a PPFD value
Every phone has a tiny built-in sensor called an ambient light sensor (ALS), whose main job is to dim the screen at night and brighten it in the sun. On Android, apps can simply read the sensor's value. This value can then be multiplied by a factor for the lamp type and presented as PPFD. And this is exactly what PPFD meter apps on Android do.
All except for Photone.
Photone is fundamentally different. It uses the front camera as the sensor and evaluates the raw pixel data that contains all wavelengths from 400 to 700 nm. Every supported phone model is measured and calibrated against NIST-traceable reference equipment, so the same light gives the same number on every phone. Photone's measurement engine is also designed for PAR, so it measures photons across the PAR range rather than relying on the human eye's sensitivity curve like a lux meter does. It uses the light-source setting just to correct the remaining spectral mismatch.
One approach borrows a sensor built for a different job. The other turns the camera into a light meter. Both end up with "PPFD" on the screen.
Next to a $2000 PAR meter
To show what that difference looks like, we compared PPFD Meter and Photone next to a LI-COR LI-190R quantum sensor: a $2000 professional PAR meter. To compare the apps, we used phones from the latest generation of Samsung Galaxy models.
Test Setup
- Photone: version 1.5.0 (with the Diffuser & Cosine Corrector accessory, light source set to "LED full spectrum")
- PPFD Meter: version 12.05 (light source set to "LED 3000K")
- Reference: LI-COR LI-190R quantum sensor
- Light source: Samsung LM-301H LED, 3000 K
- Smartphones: 2× Galaxy S26 Ultra, Galaxy S26+, Galaxy S26, Galaxy S23 FE
Results
Summary
- Large errors in the PPFD Meter app
We saw errors ranging from -11% to +24%, with up to +90% error. In other words, in the largest case, the app showed almost twice the reference value. - The PPFD Meter app shows a different number on every phone, even with the same settings
This is the finding that matters most: a reading from that app can't be reliably compared with the same app on your friend's phone, let alone with a real meter or a lamp's spec sheet. - The price of the phone doesn't guarantee accuracy
The S26 Ultra is one of the most expensive phones on the market, and yet the PPFD Meter app showed the largest error on it. - Photone's readings remained within ±2% across the tested devices and compared to the reference
That's not luck. It's what calibration per phone model is for.
Why the PPFD Meter app is inaccurate
How can the PPFD Meter app yield measurement values that are 90% off?
Well, this is not necessarily because it was programmed badly. In fact, it seems as if the factors it applies to transform the ambient light sensor's (ALS) lux value into PPFD are carefully chosen.
The actual problem is fundamental: ALSs are built into phones to regulate display brightness — not to measure light. We have an entire article that explains why ALSs are bad light meters.
The short version is this: you can't take a value from a sensor that isn't calibrated for light measurement, may not even actually measure lux despite reporting a "lux" value, apply a conversion factor established for a single device, and expect the result to work reliably across different phones.
Beyond this test
This comparison is just a short demonstration of the issue. There is a lot more we could test: different light sources, light levels, phone models, and competing apps. A truly comprehensive comparison would take a significant amount of time. We'd rather spend that time improving Photone and continuing to build the best PAR meter app we can. But if you'd like to see a more exhaustive comparison, let us know in the comments on the YouTube video. If there is enough interest, we might just do it.
At the end of the day, putting "PPFD" on the screen is easy. Building a reliable PAR meter is a very different challenge. That's why we don't want to compete on who can display the most convincing number. We want to make sure the number actually means something.
Because an app showing a PPFD number doesn't make it a PAR meter.
