How are the current transfer ratio (CTR), trigger LED current, and threshold input current measured?

Refer to Toshiba Application Note “Glossary of Photocoupler and Photorelay Terms”, section 2. Measurement procedure for current transfer ratio, I_C/I_F(CTR) - Transistor output, section 3. Measurement procedure for threshold input current, I_FHL(I_FLH) - IC output, and section 4. Measurement procedure for trigger LED current, I_FT.
(https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/application-note.html)

Example: Current transfer ratio (CTR) measurement circuit
(Excerpts from Application Note "Glossary of Photocoupler and Photorelay Terms",
section 2. Measurement procedure for current transfer ratio,I_C/I_F(CTR)- Transistor output)