Indoor and Outdoor Lighting: The Best of Both Worlds

By Frank Rauscher
Published: January 21, 2020
Key Takeaways

Many indoor gardens aren’t set up to take advantage of natural sunlight, but doing so is practical, effective and leads to huge savings on electric bills. To get started, the first thing a grower must do is measure how much natural light he or she has access to, and then add the right amounts of artificial lighting so both work in harmony to create massive yields.

Showing restraint is important for every garden. When it comes to the light needed for good plant vigor and maximum yield, being able to think outside the box is helpful.


Often, outdoor sunshine isn’t reliable enough or just not available long enough to give the crop everything it takes to grow well. If we are growing in a cold climate or a colder time of year, putting our crops outside might not be an option. Growing indoors is a common way to remedy these issues, but often indoor gardens are not set up to take advantage of free sunshine.

Fortunately, there are ways we can make use of natural light when it is available, and add to it. To make proper use of these different light energy sources, let’s look into the ways different lights affect our plants.


What Types of Lights Do Plants Use?

Fluctuating electromagnetic radiation causes the energy we know as light. This fluctuating radiation occurs at the highest frequencies and when the frequency of this energy is within the right range, it becomes visible to our eyes as well as useful for our plants.

Consider the types (or frequencies/wavelengths) of light first. Light has many frequencies and many of these are not relevant to plant growth. We typically refer only to frequencies that we can see as “light” and those we can’t as something else.

The infrared range begins just below visible red, microwave next, and radio-frequency waves are the lowest. On the high end of the spectrum, ultraviolet is the first range beyond visible, with X-ray and gamma higher still. The frequencies that plants need are essentially the visible light range.


Whether it’s synthetic light from metal halide, high-pressure sodium, LEDs or natural sunshine, using the right kind of light in the growroom is essential. The light frequencies that benefit plants are those in the photosynthetically active radiation (PAR) and those that are in the wavelength band between 400 and 700 nanometers (one-millionth of a millimeter). This just happens to also be the visible light band.

Read More: 10 Things You Must Look for When Choosing LED Grow Lights


Of course, different crop types and stages require different amounts of this energy for optimal crop vigor and growth. Orange has a significant influence on flowers, while blue influences foliage growth. When there is not enough PAR light energy, plant health declines, or in extreme cases fails altogether. If there is too much, plants will be unusual and have unwanted effects; we need to find the right balance.

Providing Optimal Light to Your Plants

How can we know when we have enough, or too much light in the indoor garden? A common unit for measuring light is the foot candle (FC), or the luminous flux (lux) used in Europe. This measurement provides an instant reading of light intensity or energy, but does not reflect the daily accumulation of this light or the fact that this can, and does, change.

To achieve optimal light, we need to determine the amount and bandwidth provided during a grow day, along with the hours when this is provided. We need to determine the accumulation of light energy, not just the instantaneous light.

Synthetic light energy is relatively stable, so total energy, along with bandwidth or frequency provided, can be calculated based on an instantaneous reading. More accurate than foot candles for measuring plant-needed light is a horticultural measurement called micromoles per square meter per second, which takes the amount of time the light is supplied into consideration. In order to be able to talk about this stuff without getting overly complex and beyond convenient comprehension, let’s review the important terminology.

Daily light integral (DLI) is the amount of PAR energy provided to the crop in a single day and is expressed as moles per day. Look at it like you would a rain gauge. When attempting to combine outdoor sun radiation light energy with indoor synthetic light, the DLI is important.

Read More: Beyond the Basics - PAR and DLI Light Measurements

DLI will influence flowering, stem thickness, root growth and plant quality. To get as much sunlight as possible into your garden, you’ll need to have it located near a south-facing window if possible. If you have a corner with windows to the east or west and south, that’s even better.

Light meters are a great tool for determining if you have a location with enough light and direct sun to grow healthy and vigorous plants. Instant readings from a relatively inexpensive meter can do that for you. These are also useful in determining whether the synthetic light you are providing is adequate.

You will need to have some estimate of the PAR provided naturally during a day to know how much you need to add through grow lights. This important step can reduce your power bill by as much as 50%. Of course, cloudy days won’t provide much help, but imagine how reducing your electric expenses by half could benefit your bottom line.

More complex and expensive light metering equipment like micro weather stations can also provide accumulated data on DLI. There are also more economical data logger systems that will measure ambient light.

Using associated software, readings can be recorded and reviewed to determine the approximate amount of DLI received in a day. You can even take periodic hand readings with a conventional light meter, or even a combination (light, pH and moisture) meter. These meters do not provide a DLI reading but will provide an instantaneous light reading. An estimate of DLI can later be calculated, however.

Don’t Have the Budget for a Weather Station?

Figuring out how much PAR is provided to your crop can also be done by using a simpler precision light meter and logging the readings each hour during the day. When synthetic light is being provided, it’s a good idea to sample the reading from this source also.

You have timers or clocks that control synthetic light energy, so determining the amount of this energy over time should be easy as you can count on it being consistent during activation. Nature’s sunshine, however, will vary throughout the day, and you’ll need to interpolate or calculate an average and daily total for natural light from your periodic readings. Add these two different light energy calculations together and you will arrive at the DLI.

Preparing Your Growroom

If you are serious about your growth ambitions, you can modify the room you select for your garden and add larger windows that face the right direction. Remember though, that when growing indoors and making use of outside natural sunshine, your grow area will be providing considerable humidity to the room.

Ventilation, painted walls and sealing cracks will likely be necessary to prevent damage. Adequate cleaning also prevents mold. The time required for these is not a lot compared to how much you might save in electricity. Setting up the garden initially is the big assignment.

What About UltraViolet Light?

There are three named bands of UV light: UVA, UVB and UVC. UVB and UVC have the shortest wavelengths and are generally filtered out by our ozone layer. UVA is the most relevant type of UV light energy that plants deal with. Exposure to UVA, because it has been present for eons, triggers many normal healthy reactions in plants, from disease resistance to the creation of antioxidants.

So, providing this type of light energy can be important, and naturally using sunshine does this. UVA is not part of the PAR spectrum and plants can perform photosynthesis without it, but with many plants, there will possibly be undesirable effects if it not present.

There are less expensive meters that measure the availability of UVA and UVB light energy. If the windows use a UV filtering screen on them where you plan to grow, you may want to check into what effects this could have on plants before getting too far into the project. A test grow can help with this.

Read More: Tanning Your Plants - The Curious Effects of UVA and UVB

When using sunlight as one of our significant sources of light energy and adding a synthetic source, we add these two sources together to determine our DLI. Calculate them separately and then add them together.

The amount of time these are applied to various plants will also trigger some plants to grow in a different phase, and this needs to be part of our light management plan. The type of plant and stage of cultivation will affect the determination as to what frequencies (or colors) we need to provide and at what DLI.

Depending on the amount of direct sunlight the indoor set-up will get, you can, of course, run synthetic light simultaneously with the natural. Calculating the amount of each will need to be done separately, and you might need certain synthetic lamps on different timers than others. Doing the calculations can be fun if you enjoy math!

Putting It All Together

When combining synthetic and natural light sources, determining the amount of PAR and DLI light energy will be critical to optimizing the vigor and growth of your crop. It is a good idea to measure the light the crop is getting as this can better help you understand what is affecting production. Take notes when making adjustments.

Don’t make too many at once! Otherwise you’ll have no idea which adjustment made the greatest difference in plant health and yield. Don’t be afraid to add some technology to your growth plan. It’s fun and can be beneficial.

Read Next: Amps, Watts and Volts - Preparing Your Home's Circuitry for a Growroom


Share This Article

  • Facebook
  • LinkedIn
  • Twitter

Written by Frank Rauscher | Writer, Owner of Garden Galaxy

Profile Picture of Frank Rauscher
During his many years of service in horticulture, product development and sales, Frank has performed innumerable visits to landscapes to facilitate a correction for struggling plants or assist with new design. He also writes for Southwest Trees and Turf and The Green Pages, is the owner of Garden Galaxy and manages several websites. He has four children and eight grandchildren.

Related Articles

Go back to top
Maximum Yield Logo

You must be 19 years of age or older to enter this site.

Please confirm your date of birth:

This feature requires cookies to be enabled