Best light meter for plants: Optimise cannabis growth

Your cannabis plants live or die by their light exposure.

Get it wrong, and you’ll end up with weak growth, poor yields, and wasted electricity. But without some way of measuring light, you’re just guessing.

This guide gives you the best light-sensing options if you’re growing cannabis, how to use them, and why measuring PPFD and PAR is the only way to ensure your plants get the right amount of light. 

If you don’t fix your grow lighting, your plants could suffer from light stress, slow development, or reduced potency. 

Keep reading to discover how to optimise your lighting setup for healthier, high-yield cannabis plants.

Disclaimer: Any information given on this site is for educational purposes only. Please ensure if you’re growing cannabis, you’re doing so by the law and subject to appropriate permissions and licenses of the applicable country.

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Why you need a light meter 

You might think your grow lights are giving off enough light, but without measuring, you’re just hoping. 

The human eye isn’t great at detecting light intensities, especially across different spectrums. 

That’s why professional cannabis growers use light sensors to accurately measure photosynthetically active radiation PAR and photosynthetic photon flux density PPFD. 

A lux meter can give you a general idea of light levels, but it measures brightness based on human vision, not plant growth. 

To grow cannabis, you need a PAR meter or a PPFD meter, which measures how much usable light your plants are actually absorbing.

 If your plants are overexposed to light, you’ll see bleaching, leaf stress, and wasted electricity. Too little, and your cannabis plants won’t grow properly. 

Using a meter to measure light ensures you hit the sweet spot for healthy growth and maximum yields.

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Types of light meters 

Not all light sensors for plants are the same, and choosing the right one depends on what you need to measure. 

Some meters are budget-friendly but provide limited data, while top-of-the-line models like the Grow Sensor offer everything you need, from detailed spectral analysis and long-term tracking. 

Here’s a detailed breakdown of the different types and how they work.

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1. Lux meters

✅ Cheap and widely available
✅ Measures brightness in lux or foot-candles (luminous flux per unit area)
❌ Not accurate for cannabis grow lighting 

A lux light meter for plants measures the intensity of light as the human eye perceives it. 

This makes it useful for general lighting checks but not specifically for growing cannabis, as lux meters don’t measure the PPFD or PAR range. 

Most lux meters are designed for photography, office lighting, or industrial applications, meaning they lack the precision needed for grow lights. 

However, they can still provide a basic reference to ensure your grow room isn’t too dim. Some growers use them alongside PAR meters to get an overall sense of light output.

If you do use a lux meter, be aware that different light sources (LED, HPS, CFL) emit different spectrums, and the conversion from lux to PPFD varies between them. 

That’s why lux light intensity meters for plants aren’t reliable for fine-tuning a grow setup alone.  

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2. PAR meters

✅ Analyse PAR spectrum (400–700nm) to assess light quality
✅ Helps growers determine light spectrum efficiency for plant growth
✅ Often used alongside a PPFD meter to measure light intensity accurately

A PAR light meter for plants is specifically designed to analyse the light spectrum that plants absorb for photosynthesis. 

These meters assess the balance of blue, green, and red light within the PAR range, ensuring your grow lights deliver the correct wavelengths for optimal cannabis growth.

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When to use a PAR meter

• Comparing different grow lights to determine which has the best spectral output.
• Checking for spectrum deficiencies in LED or HPS setups.
• Ensuring that your grow lights provide the right mix of blue/red light for each growth phase.

If you want accurate light intensity readings, you need a PPFD meter (quantum sensor) alongside a PAR meter. 

High-end meters, like the Grow Sensor, combine PAR spectrum analysis and PPFD measurement, making them the best kind of light meter for growing plants.   

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3. PPFD meters

✅ Measures PPFD (Photosynthetic Photon Flux Density)
✅ Provides μmol/m²/s readings for accurate plant light measurement
✅ Best for optimising grow lights for cannabis cultivation

A PPFD meter is designed to measure the amount of usable light that reaches your cannabis plants at a specific point in time. 

Unlike lux meters, which measure brightness based on human vision, PPFD meters provide readings in μmol/m²/s, the unit used to quantify photosynthetically active photons reaching the canopy.

PPFD meters allow you to determine exactly how much light intensity your plants are receiving, ensuring they get optimal exposure throughout their growth cycle. 

Using a PPFD meter, you can:

• Check light distribution – Measure PPFD at multiple points across your grow space to ensure even light coverage.
• Adjust grow lights effectively – Raise or lower LEDs, HPS, or CMH lights based on PPFD readings to avoid light stress or stretching.
• Fine-tune light intensity for each growth stage – Ensure seedlings, vegetative plants, and flowering plants receive the correct PPFD levels.

A PPFD meter is one of the most valuable tools for cannabis growers. 

It helps ensure your plants receive the right amount of light for strong, healthy growth and bigger yields. 

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Are there light meters that measure both PAR and PPFD?

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1. Apogee MQ-610 Extended PAR Meter (approx £600)

• Measures PPFD in μmol/m²/s
• Analyses PAR and E-PAR spectrum (400–750nm, includes far-red light)
• Ideal for LED, HPS, and CMH grow lights

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2. LI-COR LI-250A Light Meter with LI-190R Quantum Sensor (approx £600)

• Provides accurate PPFD readings
• Measures PAR spectrum for grow light evaluation
• Used in scientific and agricultural research

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3. Grow Sensor (£599)

• Tracks DLI (Daily Light Integral), E-DLI, and PPFD
• Measures Extended PAR (E-PAR) spectrum, including far-red light
• Logs data trends for precise grow light adjustments
• Is the complete environmental monitoring tool monitoring 20 key metrics!

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These all-in-one meters are the best choice for serious cannabis growers who want to optimise both light intensity and spectrum for maximum yield.

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4. Digital light meters

✅ Measures DLI (Daily Light Integral)
✅ Tracks how much light exposure over time
✅ Some models include full-spectrum analysis

‍A digital light meter for plants goes beyond single PPFD readings by tracking light exposure over long periods. 

One of the most advanced features of these meters is their ability to measure DLI (Daily Light Integral), which shows how much light your plants receive over an entire day.

DLI is particularly useful for growers who are using automated lighting schedules. Instead of checking light levels manually, a DLI meter records the total amount of PAR received over time. 

This allows you to fine-tune your light cycle to avoid too much light exposure or inconsistent light distribution.

High-end models like the Grow Sensor offer even more features, including E-PAR (Extended PAR) tracking, which measures wavelengths beyond traditional PAR, giving you a more complete picture of how your grow lights interact with your cannabis plants.

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5. Light meter apps 

✅ Convenient and cheap
✅ Some apps claim to measure PPFD
❌ Not as accurate as dedicated meters

With the rise of smartphone apps, many growers ask whether a light meter for plant apps can replace a dedicated PAR meter. The answer is not entirely. 

Apps like Photone Grow Light Meter can provide rough lux or foot-candle readings, and some meter apps claim to measure PPFD, but they lack the specialised sensors needed for accurate light spectrum analysis.

Phone cameras are designed for human vision, not for detecting PAR range. 

This means that while light sensing apps for plants can give a basic estimate, they shouldn’t be relied on for serious cannabis grows. 

If you’re on a budget, they can help with general light adjustments, but a PAR meter is the better option for precise tuning.

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Best light meters for cannabis 

Choosing the best light meter for indoor plants can make the difference between average and exceptional cannabis yields. 

Whether you're fine-tuning a grow room or ensuring your plants receive the right amount of light, having a reliable meter is essential. 

Below are some of the best options, each offering a unique set of features to help optimise lighting for your cannabis grow. 

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1. Apogee MQ-600 PAR Meter – Best for precision and professional use

✅ Highly accurate PPFD meter
✅ Measures PPFD and DLI
✅ Used by professional growers

The Apogee MQ-600 is one of the most precise PPFD meters available, making it a top choice for serious cannabis growers who need accurate light intensity measurements.

Unlike lux meters, which measure brightness as perceived by the human eye, the MQ-600 directly measures PPFD (Photosynthetic Photon Flux Density) in μmol/m²/s. 

This tells you the amount of photosynthetically active light hitting all the plants plants, ensuring you can fine-tune grow light placement and intensity. 

Another key feature is its ability to track Daily Light Integral (DLI), which calculates how much light your plants receive in a 24-hour period. 

By monitoring DLI trends, you can optimise light cycles to prevent both underexposure and excessive light stress.  

What the Apogee MQ-600 does not measure

• It does not provide full PAR spectrum analysis (i.e., it doesn’t break down wavelength distribution).
• It does not measure extended PAR (E-PAR, 400–750 nm) for far-red spectrum tracking.
• It only measures PPFD, meaning it tells you light intensity, not the composition of light wavelengths.

If you need a device that analyses the full PAR spectrum (showing the distribution of blue, green, and red light) or measures far-red light, consider the Grow Sensor. It offers all that and much more, so you get more bang for your buck.

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2. Grow Sensor – Best for long-term light tracking

✅ Tracks DLI, E-DLI, and PPFD
✅ Measures Extended PAR (E-PAR)
✅ Logs data trends for better analysis

The Grow Sensor is an advanced digital light sensor for plants that goes beyond basic PPFD readings. 

One of its key features is its ability to track DLI over time, giving you insight into how much total light exposure your cannabis plants receive each day.

Unlike standard PAR meters, the Grow Sensor measures E-PAR (Extended PAR), which includes far-red wavelengths beyond the traditional PAR spectrum. 

This is useful for growers using LED grow lights, as many modern fixtures emit far-red light, which can influence flowering response and bud development.

Another advantage is its data logging capability. You can identify trends, adjust your light schedules, and make informed decisions about grow room adjustments by tracking light fluctuations. 

Instead of taking manual readings at different times of the day, the Grow Sensor does the work for you, ensuring your plants are never light-stressed.

The Grow Sensor is an excellent choice for growers who want deeper insights into how their grow lights perform. 

It’s also less expensive than the Apogee MQ-60 and just as accurate, making it best suited for dedicated cultivators looking for long-term tracking solutions.

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Additional environmental data tracked by the Grow Sensor

Beyond light-related measurements, the Grow Sensor also monitors several critical environmental factors that impact cannabis health, growth rates, and yield potential.

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1. Temperature monitoring

• Tracks ambient grow room temperature to prevent heat stress.
• Measures canopy temperature, helping to ensure plants are in the optimal range for photosynthesis and transpiration.
• Allows growers to adjust fan speeds and HVAC settings for consistent climate control.

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2. Humidity & vapour pressure deficit (VPD)

• Monitors relative humidity (RH), which is crucial in nutrient uptake and transpiration.
• Calculates VPD (Vapour Pressure Deficit), helping growers determine if plants are in the ideal transpiration zone.
• Helps prevent issues like excess moisture leading to mould or low humidity causing slow growth.

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3. CO₂ levels

• Tracks CO₂ concentration in the grow space, allowing for proper CO₂ supplementation.
• Helps maintain optimal photosynthetic efficiency when using high PPFD levels.
• Avoids wasted CO₂ enrichment, ensuring levels are kept at a cost-effective range.

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4. Root zone conditions (If used with compatible probes)

• Monitors soil or substrate temperature, which affects root function and nutrient uptake.
• Tracks moisture levels to prevent overwatering or underwatering.
• Helps optimise nutrient solution EC (electrical conductivity) and pH balance.

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3. Photone Grow Light Meter App – Best budget option for quick light checks

✅ Affordable and easy to use
✅ Best light sensing app for plants
❌ Not as accurate as a dedicated PAR meter

If you need a quick, budget-friendly way to check light intensity, the Photone Grow Light Meter App is one of the best light meter apps for plants that claim to measure PPFD. 

While not as precise as a dedicated PAR meter, it provides reasonably accurate estimates of lux, foot-candles, and PPFD levels. 

The app uses your phone's camera sensor to measure light levels. 

While this is a convenient alternative for growers who don’t want to invest in an expensive light sensor, it does come with some limitations. 

Smartphones aren’t designed for scientific light measurements, so results can vary depending on your device and light source.

That said, if you’re looking for an entry-level tool to gauge whether your cannabis plants are getting enough light, the Photone app is a decent option.

It's especially useful for checking light uniformity across your grow area or ensuring your canopy isn’t suffering from light stress.

‍However, if you're relying on data-driven decisions to fine-tune your grow lighting, a dedicated PAR meter will always be the more reliable choice.

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How to use a light meter for plants effectively 

Proper use of a light sensor is key to dialling in your grow lighting and ensuring your cannabis plants receive the optimal amount of light. 

Simply owning a PAR meter or DLI meter isn’t enough - you need to know how to take readings correctly and adjust your setup based on the data. 

Here’s how to get the most out of your light sensor.

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1. Measure PPFD at the canopy

Your canopy level is where your plants actually receive light, so you should take PPFD (photosynthetic photon flux density) readings there. 

Hold your PAR meter at the same height as your plant tops and measure in multiple locations to check light uniformity. 

If you see uneven PPFD values, you may need to reposition your grow lights or adjust their angle and spacing.

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2. Check light distribution across the grow area

Even if your PPFD readings look good in the centre, light levels often drop significantly toward the edges of the grow space. 

Move your light sensor around different areas to identify weak spots where plants might not be getting enough light. 

If readings vary too much, consider adding supplemental lighting or adjusting your light height and positioning to achieve better coverage.

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3. Adjust grow lights for optimal intensity

If your PPFD readings are too low, raise the intensity by bringing your grow lights closer to the canopy. 

However, if PPFD is too high, you risk light stress or bleaching, so increasing the light-to-canopy distance is necessary. 

‍Be mindful of intensity changes as your plants grow - what works during the vegetative phase may need adjusting in flowering.

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4. Track DLI for consistent light exposure

A DLI meter helps you monitor how much total light your plants receive over a full 24-hour cycle. 

Rather than relying on single PPFD readings, tracking DLI (Daily Light Integral) ensures your plants get consistent light exposure daily. 

This is especially useful if you’re using light schedules with varying lights on or dark hours, as it helps prevent under- or over-exposure. 

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How much light do cannabis plants need?

Getting your PPFD levels right at each growth phase is critical for maximising yield, potency, and overall plant health.  

Too little light leads to stretching and slow development, while too much can cause light stress, bleaching, or even reduced photosynthetic efficiency. 

Here’s how to fine-tune your grow lighting for each phase.

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Seedlings: 100–300 PPFD

Seedlings are delicate and highly sensitive. If exposed to too much light, they can suffer from light stress, which can lead to stunted growth or burnt leaves. 

Keeping PPFD between 100–300 ensures they receive just enough energy for root development and early leaf expansion.

At this stage, many growers use low-intensity LED lights, keeping them at a greater distance from the seedlings to prevent excess heat and stress. 

If using a light sensor, check readings at soil level or just above the leaves to ensure even distribution.

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Vegetative stage: 300–600 PPFD 

As cannabis plants transition into veg, they demand more light to support rapid growth and strong branching. 

PPFD between 300–600 encourages healthy leaf production and robust structural development without overstressing the plants.

At this stage, you can start lowering grow lights and increasing light exposure hours, ensuring an even light spread across the entire canopy. 

A DLI meter can help track whether your plants are receiving enough total light per day.

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Flowering stage: 600–900+ PPFD

During flowering, cannabis plants need higher PPFD levels to maximise bud production and resin development. 

A range of 600–1000 PPFD ensures your plants get enough light to fuel photosynthesis while avoiding light stress.

If pushing towards the higher end (900+ PPFD), ensure your plants receive CO₂ supplementation, as this helps them process higher light intensities more efficiently. 

Without added CO₂, plants may struggle to utilise excess light, leading to diminishing returns.

By tracking PPFD levels with a PAR meter, you can optimise light intensity for each phase, ensuring consistent, high-yielding results.

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Avoiding too much or too little light 

Balancing the intensity of light is key to maximising yield and preventing stress-related issues in cannabis plants. 

Excessive light can cause bleaching, leaf burn, and reduced photosynthetic efficiency in most strains, while too little light results in slow growth, stretching, and weak bud development.

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Signs of too much light 

• If your PPFD levels are too high, plants will show bleaching on upper leaves, especially near the light source. 
• You may also notice curling leaves, dry tips, or slowed growth. 
• This happens because excessive light intensity can overwhelm the plant’s ability to process photosynthetic energy.

If you see these signs, raise your grow lights or reduce the duration of light exposure.

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Signs of too little light

• When plants don’t receive enough light, they become tall and spindly, stretching towards the light source. 
• Lower leaves may yellow and drop off prematurely, and bud sites will form loosely rather than compactly. 

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If your light sensor shows low PPFD readings, bring the lights closer or add supplemental lighting.

By consistently measuring PPFD and DLI, you can fine-tune light intensity to hit the sweet spot - maximising growth while avoiding light stress.

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Takeaways

Investing in a high-quality light sensor for plants is one of the most important steps you can take to optimise your cannabis grow. 

Light is the driving force behind photosynthesis, and without knowing exactly how much light intensity your plants receive, you’re left guessing - which can lead to underwhelming yields, slow growth, or light stress.

A PAR meter is essential for accurate PPFD readings, allowing you to adjust your grow lights based on measurable data rather than assumptions. 

Beyond just measuring light intensity, tracking DLI can give you a more complete picture of your plant’s light exposure over time. 

This ensures that even as your plants grow and canopy coverage changes, they still receive the optimal amount of light for healthy growth and high yields.

If you’re serious about cannabis cultivation, a tool like the Grow Sensor will help you fine-tune light levels throughout every stage of growth, ensuring maximum efficiency and yield potential.

For a similar cost, the Grow Sensor measures the same light metrics as precision Apogee equipment PLUS many other crucial environmental factors to help you optimise your grow.

At the end of the day, successful cannabis cultivation is about consistency and precision. 

A reliable light sensor ensures you’re not wasting energy on inefficient lighting setups, money on electricity bills or limiting your yield potential due to light deficiencies. 

Happy growing! 

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FAQs

How much lux do plants need?

The lux needed for plants depends on the growth stage and plant type. 

For cannabis plants, recommended lux levels are:

• Seedlings: 5,000 - 15,000 lux
• Vegetative phase: 15,000 - 40,000 lux
• Flowering stage: 40,000 - 70,000 lux

However, lux meters measure the luminous flux per unit area as perceived by the human eye, not plants.  

For accurate measurement, use a PAR meter to measure PPFD (μmol/m²/s) instead.

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Is there a PAR meter app?

Yes, there are PAR meter apps. While these apps can estimate PPFD readings, they rely on your phone’s camera sensor, which isn’t as accurate as a dedicated PAR meter. 

For precise measurements, a professional PAR light meter is recommended.

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