The term, "Roast Degree" pertains to how far along in the roasting process, a batch of roasted coffee beans are. 

Although there are a number of physical and chemical changes that occur during the roasting process, the easiest and most practical way to gauge how far along in the roasting process they are, is by measuring the color of the roasted beans.

Color has traditionally been measured by batch sampling. This is because at the time the current standards were developed, the technology to measure color continuously, while roasting, was simply not available.  
COFFEE COLOR AND ROAST DEGREE
There are a number of different methods, and instruments available, that measure color effectively. If you are roasting for your own customers, a simple color chart, or inexpensive color analyzer may suffice. However, if you are private label roasting and blending coffee, or if you have multiple roasting facilities, you may be restricted to rank and file technology, as a matter of practicality.
While it is no longer completely necessary to sample batches during process roasting, it is practical to use this discipline when sample roasting. Here, small (100 gram) batches are roasted to create roast profiles, where  different time, temperature, and color parameters, are applied to various green coffee bean samples. 

It has been well documented that higher temperature, and shorter term roasts, will yield roasted beans with larger bean size, as well as an increase in the number and size of pores. This is important, because more and larger pores, allow for accelerated oil and moisture migration, which affects not only taste, but storage time and method considerations.
So it is not only possible, but more so a reality, that the same sample can be roasted to the same color specifications, but produce roasted coffee with different physical and chemical characteristics, by altering the temperature and time parameters. Therefore, in order to repeat a roast profile in processing, the time and temperature used to reach a specified color, are all variables that need to be logically accounted for. Most roast profile software allows for programming the green beans physical characteristics in, because obviously more wet or dry beans, even from the same batch, will require slightly different roasting conditions to reach the same point. 
While it is in theory possible to detect reflection differences between 
beans with the same color, but slightly different physical properties, (perhaps by the amount of reflection difference caused by oil migration) it is more practical and useful (The migration of oils to the surface is an ongoing process, and would require specific time constraints that may prove difficult to employ) to categorize profiles by the time and temperature parameters used to reach the target color. 
The ColorTrack Benchtop System (pictured above), is actually a modified, inline, continuous reflection measurement, analyzer. The nature of it's technology fixes a lot of the problems normally associated with measuring color in roasted coffee beans.

The ColorTrack utilizes a rapid fire laser module, that operates specifically in the 785 nanometer range of the color spectrum, which is the optimal range for sensing shade differences. 

It is relatively impervious to ambient light, as well as much less sensitive to distance, thereby eliminating most if not all of the sample preparation and human error problems associated with roasted coffee color analysis. 

The sample is rotated automatically on a turn table, so that the 
entire path of the laser transmission is measured. The results
are then collected by a data acquisition module, and converted to a digital signal, which is then expressed in a statistical format with graphic representation.

Assuming there is a direct correlation between the average color of the exterior of the roasted bean, and the ground sample, it may in time become unneccessary to sample in batches. In fact, FreshRoast Systems Ventless Roasters, currently employ real time color technology succesfully in it's roasting process.
COLORTRACK BENCHTOP ANALYZER
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New inexpensive technologies, provide accurate color analysis for a fraction of the cost of traditional methods. Here a new LED Based Analyzer makes its' way in to the coffee arena.
Here, a real time laser color analyzer is pointed towards the viewing window, where the beam
monitors the coffee bean color inside the machine, while roasting.