How to Measure Light to Ensure Your Houseplants Thrive

Ensuring that houseplants receive the correct amount of light is crucial for their health and growth. Light intensity directly affects photosynthesis, the process by which plants convert light into energy. However, determining if plants are getting just the right amount of light can be challenging, especially indoors.

Signs of Houseplants Receiving Excessive or Insufficient Light

Houseplants receiving too much light may exhibit symptoms such as leaf scorching or burning, characterised by brown or crispy edges on leaves, overall wilting, or a yellowish appearance due to excessive stress from intense sunlight.

On the other hand, plants receiving too little light may display signs such as leggy growth as they stretch towards light sources, pale or faded leaf coloration resulting from decreased chlorophyll production, reduced foliage density, or slower growth rates. Additionally, plants may lean or tilt excessively towards light sources in an attempt to maximise light absorption, indicating inadequate light exposure.

Balancing light levels is crucial for optimal plant health, ensuring they receive sufficient energy for photosynthesis without experiencing undue stress or deprivation.

This article will explore three methods we can use to measure light intensity at home, the shadow test, a smartphone light meter app, and a proper light meter. Each of these methods can help us assess whether our plants are positioned for optimal growth.

Method 1 – Measuring Light with The Shadow Test

The shadow test is a straightforward, qualitative method for evaluating the intensity of natural daylight that filters through a window to a specific area.

Here’s how to perform it:

1. Select an Object – Choose any small opaque (non-transparent) object with a distinct shape to cast a clear shadow. This could be anything from a pencil to a small plant pot, or even your finger. The object should be uniform in shape (without fuzzy edges) to create a distinct shadow.

2. Observe the Shadow – Place the object in the area where you want to assess the light intensity, and observe the shadow it casts. This could be on the windowsill, a table, or the floor, depending on where you plan to keep your plants.

• A sharp, clearly defined shadow indicates high light intensity, suitable for plants labeled as needing “bright light” or “full sun.”

• A blurred but recognisable shadow suggests medium light intensity, suitable for plants requiring “partial shade” or “indirect light”.

• A very faint or nearly invisible shadow indicates low light intensity, suitable for plants described as suitable for “low light” conditions.

3. Evaluate and Adjust – Use the results to decide if the spot meets your plant’s light needs. You may need to move the plant or adjust the light conditions to provide the best environment for its growth. If the shadow is too sharp and your plant requires indirect light, consider using a sheer curtain to diffuse the light, or moving the plant to a less brightly lit spot.

4. Monitor Plant Health – After placing your plant in the selected spot, keep an eye on it for a few weeks. Look for signs of too much light or too little light, and adjust the plant’s location as needed.

Additional Tips

• Time of Day – Perform the shadow test at different times throughout the day to get a sense of how light intensity changes. This is especially important for locations near windows facing the east (which receive milder morning sun) or west (which receive harsher afternoon sun).
• Seasonal Changes – Remember that light intensity can vary with the seasons, and a spot that’s well-lit in the summer might not provide enough light in the winter.
• Multiple Locations – It may be helpful to repeat the test in several locations around the home to find the best spots for different plants with varying light needs.

The shadow test is a practical, no-cost method to better understand the lighting conditions in your home. By matching your plants to their ideal light conditions, you can ensure they grow healthy and strong.

What Are the Limitations of the Shadow Test?

The shadow test provides qualitative rather than quantitative data. It can give a general sense of whether the sunlight coming through a window is direct (hard) or diffused (soft) but doesn’t provide precise measurements of illuminance or light intensity.

It’s also a fairly subjective test, as the interpretation of shadows can be subjective and influenced by individual perception. What one person considers a sharp shadow might be perceived differently by another.

Another factor to consider is that the quality and intensity of light can vary throughout the day and with changing seasons. A shadow test conducted at one time may not accurately represent the lighting conditions at another time.

Diffused Light vs Point Sources of Light

Important Note: The shadow test only distinguishes between concentrated sources of light and diffused (scattered) sources of light. With sunlight coming in through a window, this test can accurately tell us how strong the natural sunlight is simply because when the sun is diffused (softened) through cloud cover, it becomes less intense, and when it is direct, it’s intensity is brighter.

The shadow test does not work accurately with indoor lighting, because any point source of light (a small, concentrated, and highly directional light that emits light from a single point) such as a flashlight or bare light bulb will cast a hard shadow, even if it is dim and insufficient to support plant growth.

The reason why the sun acts as a point source of light is simply because it’s so far away, about 150 million km (93 million miles) and therefore appears as a small light relative to the size of the sky above us.

Conversely, very bright, diffused lights, such as modern LED panel ceiling lights, or diffused lighting used in photography or videography for example, don’t create shadows because all diffused lighting scatters light rays in various directions, rather than directing them in a single concentrated beam, which reduces harsh shadows by filling in the areas of darkness with softer and more even light.

Overall, while the shadow test can provide some insights into lighting conditions, it’s important to recognise its limitations and consider using more accurate methods, such as light meters for measuring the light available for indoor plant growth.

Method 2 – Using a Smartphone Light Meter App

Smartphone light meter apps provide a more quantitative approach to measuring light intensity. They usually use the phone’s light sensor that brightens dims in response to ambient light, to measure light fairly accurately. This sensor is located next to the front facing camera, on the screen side, at the top of the screen.

To use a smartphone light meter, follow these steps:

1. Download a Light Meter App – Search your smartphone’s app store for a light meter app. Many of these apps are free and can give you a good estimate of light intensity in lux (lumens per square meter).
2. Calibrate if Necessary – Some apps may require calibration using known light conditions, or against the readings of a proper light meter. Follow the app’s instructions carefully to ensure the most accurate readings. Most smartphones will be accurate enough for our purposes without needing calibration.
3. Measure the Light – Open the light meter app and hold the phone where the plant is located or where it will be placed. Make sure that the light sensor/front facing camera/screen is facing the direction of the incoming light, the direction of the window for example. The app will display the light intensity in lux. Compare this value to your plant’s light requirements to determine if the location is suitable.

Method 3 – Using a Proper Light Meter

For those who want the most accurate measurements, a dedicated light meter is the way to go.

Unlike photography light meters, which are very expensive and difficult to use, general purpose light meters are very simple and affordable. Pictured here is a UNI-T UT383 Digital Light Meter which costs under $40, and can provide readings in both lux and foot-candle.

Retail stores that sell hydroponic gardening supplies or grow lights will usually sell light meters, and some of these light meters don’t use batteries because they measure the light using silicon sensor, which are like tiny solar cells. While they’re probably not as accurate.

Here are the three silicon sensors visible on top of this analogue light meter, they last a lifetime, and no batteries required, which some people find appealing!

Analog hydroponic light meters using solar cells can accurately measure light levels in gardening setups. Yet, they may not be as accurate as digital light meters, particularly the higher-quality models, which offer greater precision and a broader measurement range. Nevertheless, analog light meters remain a valuable alternative for gardeners, providing useful information and a more affordable option.

Here’s how to use one:

1. Select a Light Meter – Purchase or borrow a general purpose light meter or one designed for gardening. These meters offer precise readings and are often used by professionals.
2. Take a Reading – Place the light meter at the plant’s location, ensuring it’s positioned where the plant receives light. Note the reading in lux or foot-candles, depending on the meter.
3. Interpret the Results – Consult a guide (such as the one shown below in this article) to determine the light needs of your plant in terms of lux or foot-candles. Adjust the plant’s location accordingly to match its requirements.

Understanding the Units of Measurement of Light

Foot-candle and lux are both units for measuring illuminance, which is the amount of light that falls onto a surface per unit area.

Illuminance describes how bright the light appears to an observer or how much light is available for a task or activity, including plant photosynthesis.

1. Foot-candle (fc) is a unit of illuminance commonly used in the United States and other countries that use imperial units. It is defined as the illuminance produced by one lumen of luminous flux evenly distributed over a one square foot area. Mathematically, 1 foot-candle is equal to 1 lumen per square foot.
2. Lux (lx) is the metric unit of illuminance used in the International System of Units (SI). It is defined as the illuminance produced by one lumen of luminous flux evenly distributed over a one square meter area. Mathematically, 1 lux is equal to 1 lumen per square meter.

In simpler terms, , the measurement of one foot-candle is the illuminance (light) produced by a standard candle that is one foot away from the subject. Both foot-candles and lux measure how much light is present on a surface. The main difference lies in the units of area used in their definitions – square foot for foot-candles and square meter for lux. Lux is the preferred unit of illuminance in most countries due to its alignment with the metric system.

To convert between foot-candles and lux, we can use the following conversion factors:

• 1 foot-candle (fc) is approximately equal to 10.764 lux.
• 1 lux is approximately equal to 0.0929 foot-candles.
• So, to convert from foot-candles to lux, multiply the foot-candle value by 10.764.
• To convert from lux to foot-candles, divide the lux value by 10.764.

Understanding Houseplant Light Requirements

Now that we have covered the methods of measuring light, it’s essential to understand that indoor plants have varied light requirements. These are often categorised as low, medium, or high light, as listed below. The right amount of light ensures that your plants not only survive but thrive.

Low Light Houseplants (50 to 250 lux)

Low light plants are ideal for areas away from windows or in rooms with minimal natural light. They thrive in shadowy conditions where other plants might struggle.

• Cast Iron Plant (Aspidistra elatior): 50-150 lux
• Chinese Evergreen (Aglaonema): 50-200 lux
• Dracaena: 75-250 lux
• Heartleaf Philodendron (Philodendron scandens): 75-250 lux
• Lucky Bamboo (Dracaena sanderiana): 50-250 lux
• Peace Lily (Spathiphyllum): 50-200 lux
• Philodendron: 75-200 lux
• Pothos (Epipremnum aureum): 100-250 lux
• Rubber Plant (Ficus elastica): 50-250 lux
• Snake Plant (Sansevieria): 50-200 lux
• Spider Plant (Chlorophytum comosum): 50-250 lux
• ZZ Plant (Zamioculcas zamiifolia): 50-250 lux

Medium Light Houseplants (250 to 1,000 lux)

Medium light plants require more light than low light plants but do not need direct sunlight. They are perfect for locations receiving indirect sunlight such as near east- or west-facing windows.

• African Violet (Saintpaulia): 500-800 lux
• Anthurium: 250-1,000 lux
• Bird’s Nest Fern (Asplenium nidus): 200-500 lux
• Boston Fern (Nephrolepis exaltata): 250-500 lux
• Calathea: 250-750 lux
• Dieffenbachia (Dumb Cane): 250-750 lux
• Fiddle Leaf Fig (Ficus lyrata): 250-1,000 lux
• Fittonia (Nerve Plant): 150-400 lux
• Kentia Palm (Howea forsteriana): 200-400 lux
• Maidenhair Fern (Adiantum): 200-500 lux
• Moth Orchid (Phalaenopsis): 200-1,000 lux
• Parlor Palm (Chamaedorea elegans): 200-1,000 lux
• Peace Lily (Spathiphyllum): 200-500 lux (also thrives in low light)
• Peperomia: 250-1,000 lux
• Prayer Plant (Maranta leuconeura): 250-750 lux
• Swiss Cheese Plant (Monstera deliciosa): 250-1,000 lux

High Light Houseplants (1,000 lux and above)

High light plants are suited for areas that receive plenty of direct sunlight. They thrive in bright environments and benefit from several hours of sunlight each day.

• Aloe Vera: 1,500-3,000 lux. Aloe Vera plants are succulents that thrive in bright light, which is essential for their growth and health.
• Areca Palm (Dypsis lutescens): 1,000-2,000 lux. Although it’s a high light plant, the Areca Palm does best in bright, indirect light to avoid leaf burn.
• Bird of Paradise (Strelitzia reginae): 1,500-5,000+ lux. This tropical plant requires bright light to produce its large, ornate flowers and glossy leaves.
• Cactus (Various species): 2,000-5,000+ lux
• Croton (Codiaeum variegatum): 1,000-2,000 lux. Crotons need high light levels to maintain their vibrant leaf coloration, though direct sunlight can sometimes cause leaf scorch.
• Geranium (Pelargonium): 1,000-2,000 lux. Geraniums bloom best with plenty of light, making them ideal for sunny windowsills.
• Hibiscus: 1,500-5,000+ lux. Hibiscus flowers need lots of bright light to bloom abundantly but should be monitored for signs of too much direct sunlight, which can damage the leaves.
• Jade Plant (Crassula ovata): 1,000-2,000 lux. While jade plants can survive in lower light, they prefer bright light to maintain their characteristic robust growth and prevent etiolation.
• Ponytail Palm (Beaucarnea recurvata): 1,000-2,000 lux. The Ponytail Palm is a drought-tolerant plant that does well in bright light environments, contributing to its water-storing trunk’s health and growth.
• Sago Palm (Cycas revoluta): 1,500-2,500 lux. Sago Palms prefer bright, indirect light but can tolerate higher light intensities.
• Succulents (Various species, including Echeveria and Aloe): 1,500-2,500 lux for Aloe and similar for most Echeveria. Many succulents thrive in direct sunlight and can tolerate high light levels, though some may prefer slightly lower intensities to avoid scorching.
• Yucca: 1,500-2,500 lux. Yucca plants are adaptable but grow best in bright, direct light, which helps them develop their thick, woody stems and pointed leaves.

These lists offer a range of plants for every lighting condition within a home, allowing us to beautify any space regardless of its natural light availability. It’s essential to research specific care requirements for each plant, as factors like watering, humidity, and temperature also play crucial roles in indoor plant health and growth.

In conclusion, measuring light intensity is a vital step in ensuring the health of our houseplants. Whether we prefer the simplicity of the shadow test, the convenience of a smartphone app, or the accuracy of a dedicated light meter, each method can help us make informed decisions about where to place our plants. By understanding and adjusting our plants’ light needs, we can create a thriving indoor garden that brings beauty and life into the home.