How do you go from watts to lumens? let's say you have a light source and using a solar panel or photoelectric diode you can absorb all the emitted light, which would produce some amount of power outputted by the solar panel. How would you then calculate the lumens? and I'm not talking about some rule-of-thumb calculation like every 10 watts is 80 lumens.
 A: That's a two part process:

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*The solar panel produces a certain wattage output


*The electical energy is converted to light energy
Simplisticly solar panels are rated in efficiency at specific temperature and light path conditions. This is provided by all reputable manufacturers and measured in Watts output per 1000 Watts insolation under specified conditions.  If desired you can go into much more detail regarding response across the spectrum, efficiencies at various temperature and more, BUT in most cases a simple Watts per kW of insolation under specified conditions is adequate.
Typical figues are in the 17%-22% range for conversion of solar to electrical energy.

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*Wikipedia AM1.5 air mass specification


*NREL Even more detail on conditions


*Buy your own miniatute test sun here


*Much more - image search here
Light output in lumen from a given radiator (typically an LED) is also specified by manufacturers - usually in lumen per electrical watt input. This too is under specified conditions. Typically either at 20 degrees C - which is almost always lower than actual die temperature due to electrical heating or, for more modern lighting orientated devices, at say 90C or 105C - which corresponds more closefly to actual die temperatures during operation in higher power LEDs. Output is also at specified voltage and cuttent input - and this is often at values lower or substantially lower than maximum ratings.
Typical modern "white" LEDS have outputs in the 120 - 200 lumen per Watt range. Some more recent devices have outputs of 220 lumen/watt and a very few are higher.
Lumen ratings are dependent on wavelength as they are relatd to eye response. A deep-blue near UV source will have extremely low lumen/watt ratings and output at such short wavelengths is often specified in mW of light energy.
Maximum lumen per mW are achieved at the peak spectral response wavelength of the human eye - nominally 550nm wavelength. This is a "sort of yellowy green colour" - and the reason why some fire-engines are coloued "sort of yellowy green" rather than red.

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*Lumen per Watt - image search. Each image is a related webpage here

The relative sensitivity of the (standardised) human eye at various wavelengths of light is shown in the image below.
This image is from this page Sensitivity of the human eye
To avoid confusion, I recommend starting with considering light bright enough to see colours.
The photopic curve relates to light above about 5 mW per square metre - where the "cones" in the centre of the eye give colour vision. This is about the level of light in a dimly lit corner of a room with a single central light bulb.
The scotopic curve relates to very low level light where the rods in the periphery of the eye give mainly monochrome vision.
Peak bright light response is defined as being 683 lumen for a source of 1 Watt of 550 nanometre light on one square metre. At very low light levels the eye operates in a monochrome viewing mode and somewhat different resuslts are obtained.

Lumen / Lux / Candella comparison here
Useful What is lumens? How to Choose the Right Lighting Lumens?
And Response of the eye to light
