Different plants need different amounts of light.
Generalizing, we have four kinds of light at our disposal. From weakest to strongest they are:
- Ambient indoor lighting
- Indoor window light (ie, light which has passed through a window)
- Outdoor natural light (season permitting)
- Grow lighting
Plants benefit from specific parts of the light spectrum:
Ambient light can be almost totally ineffective both because it isn’t as concentrated as natural light and because artificial lighting often uses all the wrong wavelengths (since the design goal of artificial lighting is to use the minimal energy necessary to provide people with visibility).
Window light is still inferior to outdoor light for plants because windows don’t transmit all of the light and certain windows (low-E) are designed to filter out some parts of the light spectrum. Plants in windows can also get less light because of the direction of the window and the limited angle light can come in at. Put plants in a south facing window, since we’re in the northern hemisphere. If your window gets weak light, you can boost it a little by building or buying a reflector to place behind a plant and reflect window light back to the plant. In my window I’ve grown chili plants, tomatoes, and herbs, but I haven’t had luck with greens like lettuce and spinach yet even though they’re supposed to like a slightly cooler, shadier location.
Outdoor lighting is great to take advantage of in the summer! Build systems that are outdoors-proofed and set them up outside. Be extra careful to avoid materials like vinyl and PVC which are not UV stable and start to break down and leech chemicals when exposed to UV light. Two tips: Even for a ‘full sun’ plant the ideal location will give a little shade protection in the very hottest part of the day, and in extreme heat some people will add ice cubes to the reservoir to chill the water
To grow more demanding plants without natural light, grow lights are necessary. Where regular lights maximize the parts of the light spectrum that improve our visibility, grow lights maximize their light output at the frequencies which plants benefit from, as shown above. In particular, plants benefit from blue light at around 450nm in their vegetative growth phase (ie, when they’re making green stuff) and 670nm in their flowering and fruiting phase (verify these numbers). This website has an excellent overview of grow light types, though I will add a couple things.
This is a helpful diagram for visualizing the temperatures of different grow lights:
Finally, I think there’s excellent potential for LED lights, just that commercial options are not great yet. LED lights don’t heat up as much as other types, use less energy, and last an extremely long time, with the downside being that they’re expensive (at least if bought commercially) and that the most efficient ones are visibly red and blue which may not be attractive.