Off Topic: A light source to use a prism to project a rainbow on a wall.

[paulca]

I took the brightest light I had, granted an LED grow light, put it in a box with a slit 1mm wide cut in it.  Curtains closed and we could barely make out the rainbow at 6 inches from the prism on a sheet of card.

Obviously I need a focused flat beam of light or something.

Can any of you, of wide engineering fields, suggest something that could be DIY'd and not cost $100s of dollars?

BTW, I figured a prism was the "purist" way to do this, but I'm open to other suggestions.

As a Plan Z it's a strip of addressable LEDs across the skirten board shining up the wall.

[Someone]

I took the brightest light I had, granted an LED grow light, put it in a box with a slit 1mm wide cut in it.  Curtains closed and we could barely make out the rainbow at 6 inches from the prism on a sheet of card.

Obviously I need a focused flat beam of light or something.

Can any of you, of wide engineering fields, suggest something that could be DIY'd and not cost $100s of dollars?

BTW, I figured a prism was the "purist" way to do this, but I'm open to other suggestions.

A slit is attenuating (most) of the light, so you're really fighting with physics there. The general problem is the illumination in the rest of the room so how bight does the rainbow need to be?

A narrow LED bulb can project a decent rainbow off a CD

[strawberry]

 "grow light" might be wrong source to produce rainbow in visible spectrum range

[jogri]

Don't use a slit, that's wasting like 99% of the light. You want to get as much light as possible into the prism, so something like a spotlight with a reflector and maybe a lens attached to the front (to focus the light onto the prism) is way better since that way almost all of the produced light enters the prism.

The optimal source would be something that emits black body radiation (so no LEDs, only "old school" light bulbs), but you have to keep the temperature in mind cause that thing is going to get hot. Maybe start with a white LED (check the emission spectrum beforehand) and if that doesn't look great switch to a normal light bulb.

But honestly, if you're only planning on having a rainbow across a wall a LED strip with a good diffuser sheet is probably the better option.

[paulca]

I considered the sun with the prism mounted on the window sill so the curtains could be closed and just allow the like through the prism into the room.

But without some form of lens focusing sun like into the prism and/or a lens focusing the light out of the prism, the rainbow will be barely distinguishable.  As this was for my daughters room, she asked for rainbow, I bought her a prism, I figured anything that focuses sunlight (even into a glass prism) is too dangerous for burn and even fire hazards.

Maybe the LED strip would be easier I actually have a length of old vertical blinds track which would make a nice conduit to hold the LED strip, mount the difuser and still be strong enough to protect them from being kicked or hit with the vacuum cleaner.

[strawberry]

thinner the beam the less energy it needs to burn something

[David Hess]

The needed lenses to make a collimated beam are not expensive, but you will need to study a little bit of optics to select them.

[Someone]

The optimal source would be something that emits black body radiation (so no LEDs, only "old school" light bulbs)

If the end result is a "look"/visual result then there is no need to produce invisible wavelengths (tungsten/halogen/incandescent etc) and an LED works just fine. Cheap LEDs can have a lumpy spectrum that would appear as dim parts in the rainbow, but modern high CRI LEDs have a relatively smooth spectrum and look fine.

... I know this as a I specifically checked some prisms and diffraction gratings with household sources on hand when the OP asked how to get a rainbow, narrow LED fitting + CD is probably the best option given their budget/skill.