Verwendung von Fotodioden zur Messung der Lichtintensität: Die Grundlagen der Photometrie

Ever wondered how those fancy light meters work, or why your camera knows exactly how bright the scene is? Turns out, a lot of it comes down to something pretty straightforward: photodiodes. These little devices turn light into electricity, and they’re super handy for measuring light intensity. If you’re just getting started with light measurement, or photometry as it’s called, photodiodes are a great way to dip your toes in.

I’ve tinkered with these things quite a bit over the years – from simple hobby projects to more serious setups checking ambient light in different environments. And yeah, they’re not perfect, but they get the job done reliably without too much fuss.

What Is Photometry Anyway?

Photometry is basically the science of measuring light in a way that matches how our eyes see it. It’s different from just measuring raw energy because human vision isn’t equal across all colors – we’re most sensitive to green light around 555 nm.

Key units you’ll run into:

• Luminous flux: Total “visible” light output, in lumens (lm).
• Illuminance: How much light hits a surface, in lux (lx) – that’s lumens per square meter.
• Lichtstärke: Light in a specific direction, in candela (cd).

For example, direct sunlight can hit around 100,000 lux, while a cozy living room might be 100-500 lux. Office spaces often aim for 300-500 lux on desks.

Photodiodes fit right in here because silicon ones have a response curve that’s pretty close to the human eye in the visible range, peaking around 900 nm but still strong in visible light.

How Photodiodes Work for Light Measurement

A photodiode is just a semiconductor junction that generates current when light hits it. Photons knock electrons loose, creating electron-hole pairs, and that flow is your photocurrent – directly proportional to the light intensity.

In practice, silicon PIN photodiodes are popular because the “I” layer (intrinsic) makes them more sensitive and faster. Typical responsivity for silicon is around 0.5-0.6 A/W in visible light, meaning for every watt of light power, you get about half an amp of current.

Dark current is the tiny leak even without light – usually nanoamps, but it matters for low-light stuff.

Photodiode vs. Other Sensors

Here’s a quick comparison table to clear things up:

Sensor-Typ	Profis	Nachteile	Am besten für
Fotodiode	Linear response, fast, accurate	Needs amplification for weak signals	Precise light measurement, photometry
Fototransistor	Built-in gain, higher output	Less linear, slower	Simple on/off detection
Photoresistor (LDR)	Cheap, easy to use	Slow, nonlinear	Basic hobby projects

Photodiodes win for real light intensity work because they’re linear – double the light, double the current.

Building a Basic Light Meter with a Photodiode

You don’t need fancy gear to start. A simple circuit can turn a photodiode into a decent light meter.

Simple Resistor Circuit

Connect the photodiode in reverse bias with a resistor (say 10kΩ). Measure voltage across the resistor – it’s proportional to light.

But for better results, use a transimpedance amplifier. That’s an op-amp setup that converts current to voltage cleanly.

Basic transimpedance:

• Photodiode cathode to op-amp inverting input.
• Feedback resistor (100kΩ to 1MΩ depending on range) from output to inverting input.
• Non-inverting input grounded.

Output voltage = photocurrent × feedback resistor.

I’ve built a few like this, and calibrating against a known lux meter is key. For a BPW34 photodiode with 10k resistor, rough conversion is around Lux ≈ 1333 × Vo (from real specs).

Calibration Tips

No two setups are identical, so calibrate:

1. Dark reading: Cover it, note offset.
2. Known sources: Use sunlight (~100k lux), room light, etc.
3. For visible light, silicon’s response is good, but add a filter if you want closer to eye response.

Why Si PIN Photodiodes Are Great for This

At Bee Photon, we use Si-PIN-Fotodiode arrays in many projects. They’re reliable, with good spectral response from 400-1100 nm, peaking in near-IR but solid for visible photometry.

One time, a customer was monitoring grow lights for plants – we suggested a simple PIN setup, and it worked great for tracking PAR (photosynthetically active radiation), which overlaps visible.

Another scenario: Checking uniformity in LED panels. Arrays help map intensity across areas.

Common Light Intensity Levels

To give you an idea, here’s a table of typical illuminance values:

Environment	Approximate Lux
Full moonlight	0.1-1
Street lighting at night	10-50
Indoor home lighting	100-300
Office desk	400-500
Overcast day	1,000-10,000
Direct sunlight	50,000-100,000

These are from standard photometry references – handy for testing your meter.

Advanced Tips for Better Measurements

• Reverse bias: Speeds things up, reduces capacitance.
• Temperature: Silicon response changes a bit with heat – compensate if precise.
• Noise: Use low-noise op-amps for dim light.
• Dynamic range: Switch feedback resistors for bright vs. dark.

In one project, we dealt with flickering fluorescents – averaging readings smoothed it out.

Real-World Applications

Photodiode-based light meters pop up everywhere:

• Cameras: Auto exposure.
• Environmental monitoring: UV or visible for safety.
• Industrial: Quality control in lighting manufacturing.
• Agriculture: Optimizing greenhouse light.

We’ve seen customers use our detectors for automated shading systems – measures outdoor light, adjusts blinds.

Einpacken

Using photodiodes for light intensity measurement is straightforward once you get the basics. Start simple, calibrate, and you’ll have a solid light meter.

If you’re building something or need reliable components, check out Bee Photon’s site at https://photo-detector.com/. We’ve got quality Si PIN options.

Curious about a custom setup? Drop us a line at info@photo-detector.com oder über unser Kontaktseite. We’d love to help with quotes or advice.

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