How bright do I need my LED tube practicals to be?

[Viggo Söderberg]

I'm going to be lighting a bathroom scene with LED tubes over the bathroom mirrors. I will be buying these practicals but I don't know how to make sure I'll be getting enough light for them. The coolest tubes I could find was 4000K. I want them to give off a cyan look so I will be white balancing at around 4000K, then I will be gelling these practicals with a 1/2 CTB and a Half Plus Green. (I'll also be using some orange practicals so that's why I'm not white balancing at 3200K) Now seeing as I'm gelling these tubes with two layers I'm unsure I'll be getting a lot of light. 1/2 CTB has a transmission of 52% I think, and Half Plus Green has a transmission of 82%. I want the bathroom to be pretty dark but these fixtures will be the keylight. Not sure what stop I'll be using but possible I'll be around 2.8 so that I'm getting enough light. I don't know enough about lumens to know how to calculate how strong I need these lights to be (keeping the loss of light from the gels in mind). Could I have a hand working that out maybe?

[Satsuki Murashige]

What ISO and T-stop will you be shooting at? 

Usually, I’ve found you don’t need much output from tube lights if you’re placing them so close to the subject. Also, if you want the color to be saturated, then you don’t want them too bright or they’ll look more desaturated. 

If the bathroom tiles are shiny, that will help amplify the light output as well.

[Viggo Söderberg]

4 minutes ago, Satsuki Murashige said:

What ISO and T-stop will you be shooting at? 

Usually, I’ve found you don’t need much output from tube lights if you’re placing them so close to the subject. Also, if you want the color to be saturated, then you don’t want them too bright or they’ll look more desaturated. 

If the bathroom tiles are shiny, that will help amplify the light output as well.

Think I'll be shooting at 800 ISO at 2.8. Alexa Mini with Zeiss CP.2.

[Satsuki Murashige]

24 minutes ago, Viggo Söderberg said:

Think I'll be shooting at 800 ISO at 2.8. Alexa Mini with Zeiss CP.2.

My suspicion is that you’ll be fine. 800 ISO @ f/2.8 = 12fc for normal exposure. 

[Viggo Söderberg]

10 minutes ago, Satsuki Murashige said:

My suspicion is that you’ll be fine. 800 ISO @ f/2.8 = 12fc for normal exposure. 

With any strength of the light? I was wondering what lumen value I should be looking for.

[Satsuki Murashige]

3 minutes ago, Viggo Söderberg said:

With any strength of the light? I was wondering what lumen value I should be looking for.

The old standard formula for exposure is: 100ISO @ f/2.8 = 100fc (footcandles). 

1 footcandle = 1 lumen/sq ft. 

https://www.standardpro.com/how-to-measure-light/