MEASURE (Scantraq) Standard Files

There are often questions about MEASURE Standard files. A Standard is what the Reference scan is known to correspond to. Since a standard cannot be measured the Scan button is disabled in Standard mode.

When measuring transmittance the Reference scan is typically performed with a blank sample compartment and the Standard is Constant = 100%.

In V reflectance standards might be BK7 or fused silica for low reflectance samples and Au for high reflectance samples. In theory the user owns reference optics calibrated by a national laboratory. Each such optic would have a file or printout associated with it.

In practice the theoretical reflectance of BK7 or fused silica serves as the standard. In such cases a FilmStar user need only calculate the spectrum in DESIGN and save it in CSV format. After specifying Standard = File in Scan Method 3A, the actual file is selected in the Standard pull-down in the Scan Method 3B dialog. If the file name does not appear in the pull-down, it is probably because it was not copied from DESIGN's Spectra directory to a different MEASURE Reference Spectra directory. See File.. Configuration.. Directories. Some users merely set Constant = 4.2% and assume that is accurate enough.

When necessary, as when working from a printed list of reflectance values, a Standard file can be created in MEASURE by setting the program to Standard mode <Ctlr+D> and accessing the Standard Data Editor. This dialog can also be used for verifying Excel files, etc.

FilmStar MEASURE (Scantraq) is fairly robust as to the format of a CSV file used as standard. The original FilmStar format with description and number of entries is supported but no longer required. Data can have two or three columns as shown below. Note that wavelengths are in nm and spectral values are in the range 0-1.

Excel W,R Excel W,R,T FilmStar W,R,T
380,0.0443415
382,0.0443068
384,0.0442721
386,0.0442375
388,0.0442028
390,0.0441682
 
380,4.434146E-02,0.9556586
382,4.430679E-02,0.9556932
384,4.427213E-02,0.9557279
386,4.423748E-02,0.9557625
388,4.420283E-02,0.9557972
390,4.416819E-02,0.9558318
 
"Description"           224                       
380,4.434146E-02,0
382,4.430679E-02,0
384,4.427213E-02,0
386,4.423748E-02,0
388,4.420283E-02,0

Users seem frequently confused by PerkinElmer nomenclature, especially Auto Zero which is a hangover from pre-computer days. Auto Zero refers to zero absorbance or 100% transmittance. It is a good concept for chemists who think of zero as the absorbance of a cuvette filled with solvent. In fact, the latest PE instruments do not implement Auto Zero in firmware; it is just another name for a 100% reference scan. When Auto Zero is performed with a BK7 reference, there is the possibility of pegging the instrument at about 7% reflectance.

Click here for a paper on the subject of measurement accuracy.

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