- First clarify your technical parameters:
- Transmitter LED (choice: HSDL4230, HSDL4220, HPWT-BD00-F4000,
HPWT-BD00-E4000, HPWT-BH00-H4000, HPWT-BH00-G4000, HPWT-BD00-D4000,
HPWT-BH00-F4000, HPWT-BD00(-0000), HPWT-BH00-E4000,
HPWT-BH00-D4000, HPWT-BH00(-0000))

>> More... - Transmitter lens (choice: loupe with diameter 60, 75, 80, 90, 100, 130).
Based on real measurement of these common Chinese loupes:
- Receiver lens. (choice: loupe with diameter 60, 75, 80, 90, 100, 130).
- If the transmitter is multiple, number of parallel transmitters
- In how dense fog (visibility) you want the link to surely run. Choice: 4km visibility, 2km, 1km, 500m, 200m, 100m, 50m.

- Transmitter LED (choice: HSDL4230, HSDL4220, HPWT-BD00-F4000,
HPWT-BD00-E4000, HPWT-BH00-H4000, HPWT-BH00-G4000, HPWT-BD00-D4000,
HPWT-BH00-F4000, HPWT-BD00(-0000), HPWT-BH00-E4000,
HPWT-BH00-D4000, HPWT-BH00(-0000))
- Then determine
**Launch intensity**(dBTq*sym^-1*sr^-1) of your setup from these graphs (Fig. 10, 11, 12): PostScript or PDF. Launch intensity is a quantity that says how many photons is emitted per spatial angle unit during one bit time slot. This number is expressed in decibels above tera-photon (10^12 photons) for convenience. - Then determine from this PostScript or PDF the range of your link.

There are 72 combinations how to set up a single Ronja transmitter. Some of them are very efficient, some not so much. Four of them have been selected as recommended. See PostScript or PDF, Fig. 17

Guess some transmitter and receiver lens diameter and perform range calculation according to the above. Then adjust your guess according to the result.

Nevertheless it's recommended to have the lenses as big as conveniently possible, i. e. the best recommended combination. It is always handy to have extra reliability during fog. However in some situation big optical heads are not welcome, in which a reasonable compromise must be made.

The lenses are for Ronja like antennae for microwaves (WiFi). Gain in decibels can be assigned to them as well as to the WiFi antennae. Antenna (lens) gain is defined firly simply: it's how many times the density of the energy beam becomes bigger when you start with isotropic radiator and suddenly put the antenna over the radiator.

The gain is dependent also on LED geometry (smaller LED's are sensitive to lens' aberration which blurs them out and lowers the gain) and the wavelength (the glass of lens absorbs different wavelengths differently).

Therefore gains were measured for 3 families: red diodes (HPWT-B family), HSDL4220 infra diode and HSDL4230 infra diode. See PostScript or PDF, Fig. 3,4,5.