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Magnetic particle certification procedure

This page describes the measurement procedure used to quantify the performance of the BlueLine NDT FL5000 inspection light with various fluorescent magnetic particles. The measurement procedure addresses both the fluorescence excitation and emission.

Excitation - These measurements determine how efficiently energy is transferred from the light to the particles.

Procedure:

1. Measure the excitation spectrum for the particles. This gives a graph that shows the relative efficiency of different wavelengths of light at stimulating the fluorescence of the particles.

2. Multiply the measured excitation spectrum by the spectral output (scaled to the equivalent of 1,000 μW/cm2) of an ultraviolet light . This step accounts for the fact that some wavelengths are 'more valuable' than others at stimulating fluorescence of the particular particle.

3. Similarly, multiply the measured excitation spectrum by the spectral output of the BlueLine blue light (also scaled to the equivalent of 1,000 μW/cm2).

4. Integrate the areas under the curves resulting from steps 2 and 3. The resulting values for the ultraviolet and blue lights represent the total possible energy transfer from each of the lights into fluorescence of the particles.

5. Divide the result of step 4 for the blue light by the result for the ultraviolet. This number gives the efficiency of the blue light relative to the ultraviolet light.

Interpreting the results:

Let's say the result of the steps above comes out with the number 1.0. This means that the blue light would be expected to produce just about exactly the same amount of fluorescence as an ultraviolet light that emits the same intensity. That is, a ultraviolet light that emits 1,000 μW/cm2 and a blue light that emits 1,000 μW/cm2 would be expected to produce about the same level of fluorescence intensity in the particle. In either case, a more intense light produces more intense fluorescence.

Let's say the result was 0.80. This would mean that the blue light would be expected to produce about 80% of the level of fluorescence as an ultraviolet light that emits the same intensity. You could make up for the difference by using a more intense blue light. (That is, a blue light of 1,250 μW/cm2 would produce about the same fluorescence intensity as a ultraviolet light of 1000 μW/cm2.)

Let's say the result was 2.0. This would mean that the blue light would be expected to produce about twice as much fluorescence as an ultraviolet light that emits the same intensity.

Emission - this addresses how the fluorescence emission observed with blue light excitation compares with emission observed with ultraviolet light excitation.

Procedure:

1. Measure the emission spectrum for the particles when stimulated by excitation at 365 nm.

2. Measure the emission spectrum for the particles when stimulated by excitation at 450 nm.

3. Determine the efficiency of transmission of the emitted fluorescence through the BlueLine yellow filter glasses.

In general, the fluorescence emission spectrum of any material is independent of the wavelength of light that stimulated the fluorescence. This is only true when there is a single fluorescing pigment present. If a mixture of pigments contributes to the fluorescence, their excitation spectra are likely to be different, so putting in light at different wavelengths may excite the pigments by different amounts. The measurements in steps 1 and 2 determine if this is occurring, so we can report this to users.

Step 3 above is necessary to ensure that the fluorescence emission is not being significantly reduced in its passage through the yellow filter glasses. The yellow filter material has been carefully selected to block the reflected blue light effectively while transmitting fluorescence with high efficiency, but this needs to be verified for each particle before BlueLine will certify its suitability for use with the blue light inspection system. (While it is possible that there are other yellow glasses out there that may work with the BlueLine lights, we can only certify the glasses that we supply.)

Measurement controls

Before and after each set of excitation measurements we measure the excitation spectrum for a particular particle that has broad excitation through the ultraviolet and blue range. This provides a repeatability check to ensure that the spectrofluorometer is providing consistent results. We also measure the spectrum of the laboratory room fluorescent lights. Low pressure mercury vapor lamps have well known spectral lines, so this provides a check of the wavelength calibration of the instrument.

For each particle being tested we measure the excitation spectrum three times, using a separate sample from the particle batch for each measurement. If there are significant differences between the measurements this is indicative of a problem either with the instrument or with the particle batch, and this measurement set is not used. We then measure the background signal by directing the measurement probe at a non-fluorescent surface. In post-processing the background signal is subtracted from each of the excitation spectrum measurements and the results are then normalized and averaged. The resulting corrected excitation spectrum is what is used for the calculations described above.