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Lighting Coverage Primer

Understanding HID Lighting for Your Garden: A Comprehensive Guide​

The Basics: Wattage and Coverage​

A widespread rule of thumb for garden lighting states that you need 50 watts of High-Intensity Discharge (HID) lighting per square foot of the illuminated area. However, this rule doesn't consider the shape of your garden, so let's look at a more accurate guide:

  • A 250-watt HID will illuminate a 2' x 2' garden.
  • A 400-watt HID will illuminate a 3' x 3' garden.
  • A 600-watt HID will illuminate a 3.5' x 3.5' garden.
  • A 1000-watt HID will illuminate a 4' x 4' garden.

Debunking the Watts-per-Square-Foot/Meter (WPSF) Method​

The WPSF method assumes that the bulb's intensity is equal throughout the entire grow area, meaning each square foot/meter receives the same number of lumens. However, in reality, light intensity diminishes as you move away from the bulb. Consequently, each bulb has a limited range, beyond which growth will be compromised.

For instance, a 2' x 10' grow space would require 1000 watts following the 50 WPSF guideline. However, a 1000-watt bulb covers only an area about 5 feet across, leaving the edges of your garden poorly lit. In this case, using three 400-watt bulbs or two 600-watt bulbs would be a better choice.

Different Bulbs, Different Intensities​

Another issue with the WPSF method is that it assumes all bulbs have the same intensity. This assumption is false, as 1000 watts of HPS (High-Pressure Sodium) lighting is not the same as 1000 watts of fluorescent or incandescent lighting. Fluorescent bulbs have their lumens spread out over a long tube, making them dimmer, while incandescent bulbs have the wrong color spectrum and are also dim.

Moreover, a 1000-watt HPS bulb is not the same as four 250-watt HPS bulbs. The 250-watt bulbs lack the intensity required to penetrate a thick canopy or tall, bushy plants.

A Better Guideline for HID Lighting​

Here's a more accurate guideline for lighting your grow space with common HID bulbs:

  • 1000-watt: 4 to 5 feet across (1.3 to 1.5 meters)
  • 600-watt: 3.5 feet (1 meter)
  • 400-watt: 2.5 to 3 feet (0.8 to 0.9 meters)
  • 250-watt: 2 feet (0.6 to 0.7 meters)
  • 175-watt: small, less than 2 feet
These numbers assume you have a good reflector around your bulb and reflective wall coverings. You can increase coverage with multiple bulbs due to their overlapping effect or by using a light mover.

While this guideline may not entirely replace the WPSF method, it provides a more accurate and practical approach to understanding HID lighting for your garden.
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