opto isolator - What is "Saturated CTR" of an optocoupler?

For example, in the datasheet of 4n27: .

So what is saturated and what is non-saturated CTR?

Answer

Page 134 of your datasheet: "DC Current Transfer Ratio" and "N" for "Normalized". This is the ratio between collector current and LED current. If you vary the current through the LED, the current through the transistor will vary too:

\$\boldsymbol{NCTR} = \frac{I_C}{I_{LED}}\$

According to figure 2 - 5, measured under the following conditions: \$\boldsymbol{V_{CE}=10 \text{V}}, I_F= 10 \text{mA}\$ and at various ambient temperatures (\$T_A\$)

and

\$\boldsymbol{NCTR_{SAT}} = \frac{I_C}{I_{LED}}\$

According to figure 2 - 5, measured under the following conditions: \$\boldsymbol{V_{CE}=0.4 \text{V}}, I_F= 10 \text{mA}\$ and at various ambient temperatures (\$T_A\$)

In the latter case the transistor is in saturation, this means the collector-emitter voltage across it is so low that the transistor is unable to further increase the current by changing its conductivity (due to more light from the LED).