The original in 16-bit mode is linear 14-bit acquire using Umax Powerlook III into Photoshop 15-bit space and then cropped.

Threshold in function in gamma space gives incorrect result because the function does not take the tri-color luminous efficiency correctly into account.

The linear light level 63 in gamma space 2.2 is:

(63/255)^(1/2.2)*255 = 135.0642304

Two gamma-space threshold images are provided since the direction of rounding may be questionable.

Linear space processing

1. Open the original linear higher bit-depth image.
2. convert to 8-bit mode.
3. apply Threshold=63.
4. apply inverse gamma 2.2 (actually no need to do this since it is bitmat).
5. save as grayscale gif.

Gamma-space processing

1. Open the original linear higher bit-depth image.
2. apply inverse gamma 2.2.
3. convert to 8-bit mode.
4. apply Threshold=135 or 136.
5. save as grayscale gif.