Grow Lights: Grow Lights for Indoor Plants & Best Indoor Grow Lights
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Indoor Grow Guide for Growing Lights

Image of the best indoor grow lights for indoor plants.

Are you looking for grow light tips? Interested in indoor gardening, and wondering how to use grow lights? You’re in the right place.

Browse our grow guide below to learn how to use grow lamps to grow plants indoors!

1. Intensity

The higher intensity of light, the better your plants will grow. By increasing wattages you are increasing the intensity or quantity of light available for your plants.

As light levels increase, be sure to monitor other environmental elements, including heat, carbon dioxide, humidity, and nutrient supply.

2. Spectrum

As light intensity increases, the light spectrum becomes more important. Natural sunlight is intense and has a broad and balanced spectrum. Indoor plants require the same high intensity and broad but balanced spectrum.

3. Focus

The reflector directs the light where it is needed. Well-designed reflectors are made of quality materials that have a long service life. They deliver a high percentage of the available light precisely where it is needed, uniformly.

spectral distribution


This chart shows how the intensity of light, the quality of the spectrum and the amount of time the lights are on, can affect different aspects of plant growth.

  • Intensity mainly affects the overall yield of your crop
  • The quality of the spectrum mainly affects the quality of the plants’ structure, tastes and/or smells
  • The Photoperiod (the amount of time the plants are exposed to light) mainly affects when the plants begin their flowering/ reproductive process

Intensity, spectrum, and photoperiod all have an impact on plant growth.

As you can see from the chart above, intensity, spectrum and photoperiod also have affect all aspects of growing. As an indoor grower you must consider all these lighting aspects to be a successful grower.

The lighting industry measures light intensity in lumens. Lumens represent how intense or bright a light source is to the human eye.

For plant growth, intensity should be measured in watts (ie: 600W or 1000W lamp) or watts per square foot. Typically 40-60+ watts per square foot should be used in a garden (See the Lamp Coverage Guide – HERE)

spectral distribution

This is the human eye response curve. This chart shows you what part of the light spectrum our eyes are most receptive to. Light meters are measuring the brightness of a light source at 555 nanometers.

The spectral distribution chart is the best illustration of the quality of the light spectrum. Learn more by reading the Spectral Distribution Chart section in this grow guide.

Light spectrum is the many different wavelengths of energy produced by a light source. Light is measured in nanometers (nm). Each nanometer represents a wavelength of light or band of light energy. Visible light is the part of the spectrum from 380nm to 780nm.

spectral distribution


The spectral distribution chart is a visual representation of the light spectrum produced by a lamp. It is a graph showing the relative intensities of a light source at each wavelength.

You can use these charts to compare the energy levels of various light sources used for plant growth. They are the most practical way to compare the quality of light created by different light sources. The chart shows exactly which wavelengths of light (measured in nanometers) the plants are receiving.

spectral distribution


On the left of the chart is the percent of relative energy. The highest energy output of the light source is plotted as 100% Relative Energy. The 100% peak is used to compare the energy levels of all other wavelengths of light produced by that light source.

The bottom of the chart shows all of the wavelengths of visible light energy that the light source produces. For example, if a wavelength is at 50% relative energy, that peak has half the energy when compared to the 100% peak. Scaling each chart to 100% relative energy allows side by side comparison of light sources with different lumen ratings (intensity) or wattages. For example, a 1000W HPS lamp has more overall intensity than a 400W HPS lamp even though their spectral distribution charts are the same.

Furthermore, a 1000W EYE HORTILUX Blue has 80,000 lumens with a well balanced spectrum when compared to a 1000W EYE HORTILUX Metal Halide with 115,000 lumens. (See examples below.)

spectral distribution


Spectral distribution charts are the best way to present the quality of a grow lamp’s light spectrum. Eye HORTILUX offers several different grow lamp options to allow you to choose which spectrum best suits your growing needs. Compare our grow lamps for yourself and get the best yields out of your plants today.

View Spectral Distribution Comparison Guide

Reflector design can dramatically impact the amount of light reaching your plants. In the diagrams pictured (assuming they are all using a 1000W lamp) you can easily see how three different reflector designs can change the focus and intensity of the light.

Reflector designs for indoor grow lights.

Example 1 shows the light as more intense but only covering a small portion of the plants.

Reflector designs for indoor grow lights.

Example 2 shows the light covering more of your plants but dropping slightly in intensity.

Reflector designs for indoor grow lights.

Example 3 shows the light expanding beyond the plants allowing for wider coverage but with the least amount of intensity.

This effect is similar to changing the spray pattern of your garden hose. The quality of the material used in your reflector can also dramatically affect the intensity of the available light which hits your plant canopy. Poorer quality materials are not as reflective initially and degrade quickly. These factors reduce the amount of light that reaches your plants.

Fixture efficiency is a calculation to determine how much light exits a fixture. If a fixture has a 90% efficiency rating, that means 90% of the light produced by the lamp is leaving the fixture. The higher this number is, the better your plants will grow. However, this calculation does not represent the uniformity of light hitting the canopy of your plants.

Uniformity refers to a fixture’s ability to provide even light distribution over the canopy of your plants. Poorly-designed reflectors produce hot spots where more light is directed. Uniform light distribution allows for multiple fixtures to be used and provides consistent light levels to plants which are not directly under the light source. The charts below show how three different fixtures produce drastically different results in the uniformity of distribution for the same lamp.

One Lamp, Three Fixture Types... Three Drastically Different Results

Example of photometric light distributions for grow light fixture uniformity.

The McCree Curve represents the average photosynthetic response of plants to light energy. The McCree Curve, also known as the Plant Sensitivity Curve, begins at 360nm and extends to 760nm. This curve can be placed over a spectral distribution chart to see how well a light source can affect plant growth.

spectral distribution


Photosynthetic Active Radiation (PAR) was derived from the Mcree Curve. It is a total count of light energy (in photons) between 400nm to 700nm.

The PAR measurement is quickly becoming a popular metric of the growing power of a light source. However, the PAR measurement has two fundamental flaws.

  1. Wavelengths between 380nm to 400nm and 700nm to 880nm are excluded from the PAR measurement.
  2. All photons are weighted equally regardless of wavelength.

The Mcree Curve clearly shows plants respond to energy outside the 400nm to 700nm PAR range. Plants respond differently to energy within the PAR range.

A PAR meter only measures photons between 400nm and 700nm. As you can see in the example above, PAR does not distinguish which photons of light are present; it only counts the total amount of photons present in those nanometers.

spectral distribution spectral distribution


Below are the spectral distribution charts of two HORTILUX grow lamps with very different spectrums – the HORTILUX BLUE Metal Halide and the HORTILUX Standard Metal Halide. When the charts are compared side by side, the difference in the quality of the two spectrums becomes readily apparent. The BLUE delivers a broad, full spectrum for your plants whereas the Standard Metal Halide provides a spectrum full of peaks and valleys and areas of lower spectral output.

When comparing those two spectrums many would guess the HORTILUX BLUE has more PAR than the Standard Metal Halide. However the opposite is actually true – the Standard MH has 40% more PAR than the HORTILUX BLUE yet the BLUE is clearly the better grow lamp due to the quality of the spectrum and how closely it mimics natural sunlight. How is this possible? The Standard MH produces 115,000 lumens while the HORTILUX BLUE MH produces 80,000 lumens. Since the brightness of the light directly affects a PAR measurement, the higher the Lumens are the higher the PAR measurement will be. In essence PAR is a measurement of intensity and is in no way related to the quality of the lights spectrum.

HORTILUX BLUE Spectral Range

HORTILUX BLUE MH Grow Lamp Spectral Range

HORTILUX Standard Metal Halide Spectral Range

HORTILUX Standard Metal Halide Spectral Range

Dimming your household lights is a common practice. Many indoor growers look at dimming their grow lights as the same thing. But, this could not be further from the truth. Many growers dim their lights to reduce the amount of heat inside the grow room during various stages of plant growth. Dimming HID grow lights can be done but understand this will change the quality of the light coming from the grow lamp.

Below are examples of what happens to a klamp’s spectral distribution when the lamp is dimmed. You can see how the spectrum changes; it shrinks and portions of it may actually disappear. This change in spectral energy will have a negative effect on the quality of your plant growth. We understand the need to dim your grow lights. But understand you can get better quality plant growth from a 600W lamp running at 100% than you can from a 1000W lamp running at 75%.

The effects of dimming on spectrum and grow lamp performance

Setting your ballast to overdrive can produce frequencies that are damaging to your lamp. Damaging frequencies distort the arc stream in a lamp’s arc tube. These pictures show what happens inside the lamp’s arc tube when your ballast is in overdrive mode. This distortion causes cracking and may rupture the arc tube and destroy your lamp. Play it safe and only run your lamp at its rated wattage.

Image of the effects of overdriving a grow light.

An example of a Metal Halide lamp in overdrive.

Image of the effects of overdriving a grow light.

An example of an HPS lamp in overdrive.

The human eye might not notice any difference in light intensity or subtle shifts in spectrum over as few as six months. Your plants will.

Our research indicates that burning lamps for 12 or more hours every day diminishes both intensity and spectrum. This happens with all lamp brands, since the chemical reactions within the arc tube (which create the light and spectrum) deteriorate over time.

Man-made lamps do wear out, and plants are quick to recognize these subtle changes. For example, think of putting an outdoor vegetable garden in a semi-shady spot. Sure, the plants may grow—but not to their optimum potential.

For fuller, thicker vegetative growth and greater yield, lamp replacement should be part of a regular maintenance schedule. Keep old working lamps for troubleshooting your system or as a temporary replacement if lamps burn out.

When growing indoors, light intensity and spectrum become even more important. The quantity and quality of light are the most critical factors associated with successful plant growth. Regular lamp replacement is essential.

Lamp Type Optimal Growth Average Growth Maintenance Growth
HPS 9-10 months 11-12 months 13-16 months
Metal Halide 6-8 months 9-10 months 10-12 months
e-Start Metal Halide 9-10 months 10-11 months 11-12 months

Chart based on burning Metal halide lamps for 24 hours and burning HPS lamps for 12 hours.

The spectral distribution chart is the best way to compare light sources. Brands that do not publish spectral distribution charts should not be considered to be legitimate purveyors of lights sources for indoor plant growth.

Ultra Violet (UV) light plays a significant role in all aspects of plant growth. Plants, in their natural environments, are exposed to natural sunlight which includes UVA and UVB light. Many of the lights used for indoor plant growth produce very small amounts of UV and some produce none at all.

UV light is broken up into three sections. When looking at a HORTILUX grow lamp’s spectral distribution chart, you can see the nanometers (nm) of light below the graph. UV light is on the far left section of the spectral charts. UV consists of three sections within the electromagnetic spectrum.

UVA = 400nm-315nm
UVB = 315nm-280nm
UVC = 280nm-100nm

Natural sunlight produces these three sections of UV. However, UVC is not naturally present on earth because the Earth’s atmosphere blocks the extremely harmful light from reaching the Earth’s surface.

Lighting sources can artificially produce all three types of UV, however the components used to construct the light source, such as glass, will generally block UV emitted from the light. Check the spectral distribution chart on your grow lamps packaging to see if your light is producing any UV.

Think of what happens to your skin when you are exposed to sun for long periods of time. It begins to burn and sweat. Plants are no different. UV light activates a plant’s defense mechanisms. UV causes plants to produce oils, antioxidant vitamins and flavonoids to protect themselves from the damaging effects of UV.

These compounds produce the vibrant colors, smells and tastes of your plants. If your light source does not produce UV, you are effectively changing the color, smell and taste of your crop.

Keep in mind traditional lighting measurements such as Lumens, PAR and Kelvin temperature do not measure UV. Only a UV meter or a spectral distribution chart can provide information on a light source’s UV output.

Lumens, Kelvin Temperature, CRI, CCT, CIE Chromaticity.

All these terms are standard lighting measurements that tell us how the light looks to the HUMAN EYE. These terms do not represent the quality of a light spectrum nor do they take into account the McCree Curve.

Kelvin temperature is the unit of measure used to describe what color a lamp appears to be when it is lighted. Kelvin temperature is a lighting term used to relate the color appearance of a lit lamp to the color appearance of a glowing hot piece of metal. An example is hot steel which glows bright yellow when heated to 1200 degrees Kelvin. A lamp may be called 1200K if it looks like that same shade of yellow when lit.

Kelvin temperature has no relation to light spectrum, light quality or plant growth. It is used only to describe color appearance. The examples below show the spectral distribution charts of two lamps with the same Kelvin temperature but very different spectral outputs. So never purchase a grow lamp based upon its Kelvin temperature, especially if there is no spectral distribution chart to display the quality of the light emitted by the lamp. If you choose a lamp based on its Kelvin temperature alone, you may be depriving your plants of the wavelengths they need to reach their maximum potential. Always choose a grow lamp based on its spectrum, not its color temperature.

Kelvin has no impact on plant growth!

The Color Rendering Index (CRI) is a rating system that measures the accuracy of how a light source makes specific colors look.

CRI tells us how accurately a light source displays specific colors. They are listed below.

If a light source displays these 14 colors perfectly the corresponding CRI would equal 100. If the light source did not display any color correctly its CRI would be 0 (zero).

The CRI index was created to help companies sell products. If a light source does not accurately display colors, the carpet that you purchased in the store may look completely different at home. A red shirt may look orange or pink. A white car may look grey or cream.

Do your plants care about CRI? Only if they are buying carpet, shirts, or cars.

Some horticulture lighting brands publish the CRI values. However, CRI value has little to do with lamp spectrum or its ability to grow plants.

Fixture spacing and position above the plant vary widely. Light output and coverage is greatly dependent upon the fixture design and quality of the materials used.

Please consult the fixture manufacturer for recommendations on fixture spacing relative to plants.


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EYE HORTILUX® offers grow lamps and ballast for indoor gardening applications. Our grow lamps are made in the U.S.A.

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