You're likely here because an IES, TM-14 or EULUMDAT file you received had a web link to this page.
The photometric file you have was likely generated by Photopia, an optical design and analysis raytracing software package. Photopia is widely used in the architectural lighting industry for predicting product performance without testing a physical prototype.
Photopia (and other raytracing software) can provide a photometric file as part of their output. These programs require a complete 3D CAD model of the luminaire, as well as detailed information on the lamps and materials that are used. The simulation then traces rays from the lamps, to the materials, and out of the luminaire. If the geometry, lamps, and materials are modeled accurately, there is no reason that simulated photometry can not be as accurate as physical testing.
Time and cost. Simulating is much faster than building the product and sending it to a lab to be measured. With custom products, the manufacturer may not be prepared or have time to build and test a prototype for each design. For some standard product lines, there are so many configurations that building and testing each variation is cost and time prohibitive. Simulations are quick and give the manufacturer an easy way to test different materials, lamps and geometries to design the best product. Typically manufacturers will have a physical test done on the final part to ensure it is built correctly, but some trust Photopia enough to send simulated photometry to their customers to do layouts with.
Typically, yes. There are many factors which affect the accuracy of both simulated and tested photometry, so saying one is more or less accurate than the other would be too general. Since the purpose of simulated photometry is to predict real photometry, every effort has been made in Photopia to ensure the highest degree of accuracy. Photopia takes the following into account:
Many manufacturers rely on Photopia to predict product performance and use it in their design process to save prototype costs and time. Just as you rely on a lighting software program to predict light levels, they rely on optical simulation to predict product performance. If they did not trust Photopia, they wouldn't use it. We also have many comparisons of Photopia photometry versus lab tested photometry which always show very strong correlation.
There are also aspects of simulated photometry which may not be accurate:
Interestingly, many of these issues are very similar to issues with physical testing. When asking if you can trust any photometry, it may be less about trusting the particular method of photometry, and more about trusting the company providing the photometry. Do you think they know what they are doing and have taken care to address the issues mentioned above? Do you think they're trying to make a quick sale, or develop a relationship? Are they willing to assure the predicted light levels or retest sample fixtures if they don't seem to perform as expected?
This page covers some of the issues in measured and simulated photometry, but photometry isn't the only area of uncertainty in predicting light levels in a space. Laying out a fixture in a space model to predict light levels has its own issues. The predicted light levels could vary from installed values for any of the following reasons: