You're measuring the Illuminance and comparing it to the Luminous Emittance - two very different things.
Illuminance is the amount of light that falls on your sensor.
Luminous Emittance is the amount of light sent out by the LED.
You have to remember that the light emitted from the LED is sent out over a cone of 120°, so the closer your sensor is to the LED the more of the light will fall on it, and the less will miss it.
At the moment you're measuring a small portion of the actual light - just what shines direct on your sensor, and a small amount reflected by the inner surface of your "box".
You'd achieve the best measurement if you could ensure that 100% of the emitted light fell on 100% of the surface of your sensor, and the only real way of doing that would be to have a sensor the same dimensions as the point of light in the center of the LED.
The three requirements you listed are what every designer wants: low price, high performance, low power. My electronics professor used to say that The designer's blanket is always short: if you pull it up to cover your face your feet will be cold, and vice versa.
There are two ways of deciding on this matter:
- Budget constraint
- Specification constraint
You have to decide what's more important, then you pick your components.
I'm not an export in Raman spectroscopy, but I assume that there is a relationship between the laser bandwidth and the amount of information you can recover from this technique.
If you decide that cost is paramount you'll be forced to use a laser diode since those are cheaper. If you have a price range, say $100, you can start to shop around and try to find the best laser diode that $100 can buy.
When you find what you want, you design your system around the specific diode characteristics.
If you decide that performance is all that matters, you will do the opposite thing. Let's say that you calculated that in order to meet your specifications you need a light source with a bandwidth of 1nm. At this point you can shop around and find the cheaper laser that gives you that bandwidth. Again, at this point you can design your system around it.
This argument obviously applies to every component in your system. It wouldn't make sense to have a very expensive narrow-band laser and then use a cheap monochromator that would kill your information.
If, as you said, price is the number one priority, go with the laser diode.
Best Answer
This site has a lumens to lux calculator, given beam angle and distance.
https://www.bannerengineering.com/us/en/company/expert-insights/lux-lumens-calculator.html
Looks like ~2300 lumen would yield ~1500 lux at 45 degrees and 1.5 meter.
Might look at "tactical flashlights" like these and see what kind of LED & lenses they use to achieve 1300 lumen, go from there, use 2 of them kind of thing.