Grow lights are artificial light sources used for cultivating plants indoors when natural sunlight is insufficient. These specialized lights emit the specific energy spectrum plants require for photosynthesis. A common question is whether a grow light can effectively deliver this energy if the light must first pass through a glass barrier. Glass is not a perfectly transparent medium, meaning it inevitably reduces both the quantity and quality of light reaching the plant.
How Glass Affects Light Transmission
When light encounters a glass pane, its intensity is immediately reduced through two primary physical processes: reflection and absorption. Reflection occurs at both the front and back surfaces of the glass, causing light rays to bounce away. For standard flat glass, reflection typically accounts for an approximate 4% loss at each surface, resulting in an 8% reduction for light passing straight through. This loss is compounded when the grow light is positioned at an angle, as oblique light rays reflect more strongly.
The second mechanism, absorption, involves the glass material converting light energy into heat. Thicker glass or glass with a slight tint absorbs a greater percentage of energy, further diminishing transmission. Even clear, clean glass typically transmits only 85% to 90% of the visible light, meaning a significant portion of the output is lost before reaching the leaves.
The Specific Light Spectrum Plants Need
The concern shifts from the overall quantity of light to its specific quality, or color, once the light passes through glass. Plants rely on Photosynthetically Active Radiation (PAR), the electromagnetic spectrum between 400 and 700 nanometers. Efficient grow lights concentrate energy in the blue (400-500 nm) and red (600-700 nm) regions of the PAR spectrum, as these wavelengths are the most effective for driving photosynthesis.
Standard window glass acts as a selective filter that alters this optimized spectrum. Most common glass formulations naturally block a significant amount of ultraviolet (UV) light, which includes the lower end of the PAR spectrum. While excessive UV is harmful, some UV-A is beneficial for plant development, and its filtration can subtly affect plant morphology and pigment production. This spectral shift means the light reaching the plant is not only less intense, but its color composition differs from what the grow light was designed to deliver.
Practical Implications for Indoor Growers
Grow lights do work through glass, but with compromised efficiency that requires compensation. A pane of glass imposes an immediate and permanent reduction in the Photosynthetic Photon Flux Density (PPFD). This light reduction can range from 10% to 30%, depending on the glass type, its cleanliness, and the angle of the light source.
To counteract this unavoidable loss, growers have several strategies. Whenever possible, eliminate the glass barrier, such as by placing the grow light inside the enclosure. If glass is necessary, use specialized low-iron horticultural glass, which is manufactured to maximize light transmission, often exceeding 90%. The most effective compensation strategy is to place the grow light significantly closer to the plants to increase local light intensity and overcome the percentage loss. Maintaining a meticulously clean glass surface is also important, as dust and mineral deposits dramatically increase light loss.