How to Measure Light for Plants: Apps, Lux Meters, and PAR Devices

A light meter for plants is a device that measures the amount of light available in a specific spot to determine whether it matches the needs of a plant. Instead of guessing if a windowsill, balcony, or grow light is bright enough, a light meter gives an objective reading in lux (light intensity per square meter) or sometimes PAR (Photosynthetically Active Radiation), which is the range of light plants use for photosynthesis.

How does it work? A sensor (in a handheld device or smartphone app) detects incoming light. The reading is displayed as a number (lux, foot-candles, or PAR). The user compares that reading with the plant’s known requirements.

For example:

• A snake plant can survive in 50–500 lux.

• A monstera prefers 1,000–2,500 lux of indirect light.

• A succulent may need 20,000 lux or more of direct sunlight.

How to use this with a light meter:

• Take a reading where you plan to keep your plant.

• Compare the lux number to the category above.

• Adjust placement or add grow lights if the light is too low.

Types of Light Meters

How to Choose the Best Light Meter?

Start with Your Purpose

If you are a houseplant owner who just wants to know if a corner is too dark for a monstera or bright enough for a succulent, a basic lux meter or even a smartphone light meter app is usually enough. Apps are convenient and free, but a handheld lux meter gives more consistent readings.

If you are running a greenhouse, hydroponic system, or grow lights, accuracy becomes more critical. Plants don’t just need visible light; they rely on PAR (Photosynthetically Active Radiation, 400–700 nm) for photosynthesis. In this case, a PAR meter is the right choice, since it measures the actual light spectrum plants use.

Think About Measurement Units

• Lux/foot-candles: Fine for most home gardening. It tells you brightness levels, good for matching to general categories (low light, bright indirect, etc.).

• PAR (μmol/m²/s): Needed for precise growing environments where light intensity directly influences yield.

If your goal is healthy houseplants, lux is usually enough. If your goal is maximum harvest from crops, go for PAR.

Consider Extra Features

Some light meters do more than just measure brightness.

• Plant-focused apps: Interpret lux readings into categories like “suitable for orchids” or “too low for cactus.”

• Combo meters: Include temperature, humidity, and soil moisture sensors — helpful in greenhouses.

• Integrated smart systems: Adjust grow lights automatically, removing the guesswork.

The more control you want, the more advanced the meter should be.

Match Your Budget

• Free or under $20 → Smartphone app like AI Plant Finder or basic lux meter (good for everyday indoor plant lovers).

• $30–$100 → Higher-quality lux meters, often with calibration and stable sensors.

• $150–$500+ → PAR meters or advanced grow light systems, meant for professionals.

What to Measure

Light Intensity

This is the lux value (or PAR if using a professional meter). It tells you how much light actually reaches the leaves. You need to compare this to your plant’s category — for example, a monstera thrives at 1,000–2,000 lux, while succulents often need 20,000 lux or more.

Duration of Light (Photoperiod)

Plants need not only the right intensity but also the right number of hours of light per day. A light meter reading is just a snapshot. You should also count how many hours per day your plant receives that level of brightness. A cactus needs 10–14 hours of strong light, while a peace lily might be happy with 6–8 hours of low light.

Location Comparisons

It’s important to count different spots in your home or garden. Measure near windows, corners, and shelves. You’ll quickly see how light levels drop as you move even 1–2 meters away from a window. This helps decide exactly where to place each plant.

Seasonal Changes

Daylight shifts with the seasons. A window that provides 8,000 lux in summer might only give 2,000 lux in winter. You should count light levels at least a few times per year to see if a plant needs to be moved or supplemented with grow lights.

Artificial Light Contribution

If you use grow lights, measure directly under them to count whether they’re delivering enough lux or PAR. Distance from the lamp makes a huge difference — halving the distance can quadruple the intensity. A light meter helps you position lights correctly.

So, in practice you should count:

1. Light intensity (lux or PAR).

2. Duration of exposure (hours per day).

3. Differences by location (which spot has best light).

4. Seasonal variation (summer vs. winter).

5. Effect of grow lights (distance and placement).

Top Examples

For beginners and houseplants, AI Plant Finder is the most practical because it interprets numbers into care advice.

For intermediate growers, a basic lux meter adds precision for positioning plants and grow lights.

For professionals, a PAR meter or smart grow light system ensures maximum yield and efficiency.

“Light is the single most important factor in growing plants. You can water and fertilize perfectly, but without adequate light, plants cannot thrive.”

— Dr. Bruce Bugbee, Utah State University, Center for Plant Science.

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