Measurement Log Palette

You can access the Measurement log palette by going to window then choose measurement log.
This will open the palette that you see above.
When using either the ruler, selection or the count tool, you can click on the record measurement button and that would give you sufficient information like:
Label: automatically numbers measurements.
Date and Time: when the measure occurred.
Document: the measured document
Source: The tool used : ruler, selection or count tool
Scale : the measurement of the document you are working on.
Scale Units: the measurement unit.
Gray value : This gives the brightness value, Photoshop converts the selection into gray scale and measures the brightness according to the grayscale view.
Scale Factor: Number of pixels of the unit used for measurement.
Count: The number of counted items or measured items.
Length: linear distance of the ruler measurement.
Angle: The angle of the ruler tool.
Area: Area of selection in square units (pixels if you're still on the default)
Height : height of the selection in units (pixels if you're still on the default)
Width : width of the selection in units (pixels if you're still on the default)
Integrated Density : The sum of the pixels value of the selection
Perimeter: The total of many measurements done together.

You also have 4 buttons (starting from the left) that are:
1. Select all measurements : This would select all the measurements available on the log palette.
2. Deselect all measurements : This would deselect all the measurements available on the log palette.
3. Export selected measurements : This helps you to store the data available on the log palette and save them as a document outside Photoshop that you can keep for later need.
4. Delete measurements : This would delete the measurements from the log.