Counteract strong blue light emission from a cheap LED

Question

First of all let me tell you that I'm a real newbie in photography, I know basically nothing.

I'm trying to digitize our family slides. Just to be sure we're all talking about the same thing, I'm talking about Reversal films producing a positive image, not a negative one. It seems the term slide in photography is used for both...

I use an old slide projector with a Nikon D5000 I borrowed to my dad. The lens is a Nikkor 55-200mm that I use with a reverse ring to take macro photos.

Because the original light bulb on the projector was generating too much heat, I had issues with films popping if they were exposed more than 10 seconds in front of the light. So to avoid any more damage I replaced the original light by a cheap cool white LED.

Now here is the problem: I have blue spots appearing on the photos and I can even see with my own eyes some blue dots coming out of the LED (yes I know now this blue light is harmful for eyes, I'll be careful from now on)

I researched about this blue light and found out LEDs actually have a strong blue lighting, here is the spectrum of a classic LED

I'm aware there's a brand producing high CRI LEDs with a much better spectrum but those LEDs are much more expensive than this cheap one I bought. Here is the spectrum of this high CRI LED

So my question is: how can I avoid having these blue spots on my photos? Can I add a physical filter reducing the amount of blue light arriving to the camera but keeping other colors fine?

Or can I correct this effect with a setting from the camera? I'm looking for a solution that doesn't require manual editing on the computer as I've thousands of slides to digitize. So I'd need a solution that just works for all the pictures.

I really would like to avoid buying one of those high CRI LEDs as they're very expensive (almost $100)...

The aperture of my camera is at the smallest because of the strong direct light the camera receives and I set the White Balance of the camera to Fluorescent

Please don't forget I'm a real newbie in photography so I'm not familiar with all different kinds of settings (ISO, shutter speed, compensation...etc).

Thanks a lot for your answers

EDIT for more information:

I played a bit with the settings and randomly found something that was working:

• ISO: 800
• Shutter Speed: 1/2500
• Exposure Compensation: 0.0

I'm using DigiCamControl to control the camera.

Here is a photo of my setting (the books under the camera are just temporary, didn't have time to build a proper mount yet)

Answer 1

You appear to have actually two [& a half] separate issues caused by the same thing.

Firstly the blue spots - which, btw, won't do you any harm so long as you don't stare directly into the light a lot. The bulb appears to have several distinct LEDs inside it - not uncommon at all, & you would probably never notice in normal use, but for this particular purpose, rather a nuisance.

As the lamp runs cool, there's the possibility of an almost free method to do three tasks in one.

Tissue paper. The stuff they used to wrap fresh bread in when you bought it directly from the baker's. It's thin, it's light, it's cheap.

If you add layers of tissue between the lamp & your slide, each layer will diffuse the light more. It will also, incidentally, remove some of the blue cast overall, as it will be slightly off-white.

This brings us to your exposure & white balance.
I assume you have the camera set to the lowest ISO it can - usually 100. If not, do that first. This kind of repetitive task with known input is also one you should have the camera set to fully Manual, so you don't need to keep re-focussing [assuming you're on a fixed tripod, which you should be for this type of task] or resetting exposure etc.

We've already reduced the amount of light coming through by using the tissue paper, & also hopefully reduced the blue cast & bright spots.
All we need to do now is setup up a correct white balance. Do this manually using the tissue-covered lighting you just made.

Check the reference manual for that particular model. Instructions for the D5500 below [click to see larger]