If you have a floating point image, it expects all the values to be within the 0-1 range. If it's in that range, it will display 0 as 0 and 1 as 255. Numbers below 0 will show up as black. Numbers above 1 will show up as white.
If you use , it can take floating point numbers in any range and will display the min value as 0 and the max value as 255, and values in between will be linearly scaled in brightness.
If your display is still black even though your numbers are in the 0-1 range, it's possible all your numbers are like 0.1 or less. So a value of 0.1 will show up as gray level 25 - really dark. Lower values will also be very very dark and hard to see. If you use  then it will map the 0.1 to 255 and values in the range 0-0.1 will be linearly scaled between 0 and 255.
If your image is in the range 0-255, then you divide by 100 then your values go from 0 to 2.55. Anything 0-1 will show up in gray scale, and anything between 1.00 and 2.55 will be saturated -- clipped to 255 and appear totally white.