68 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Z % lOO 5o i CENTRE OF FIBER ß T M TMIN 0 20 z•o 60 80 pm HAIR DIAHETER Figure 7. Evaluation of a transmission profile curve. out on one cross section, one with normal light, the other with monochromatic light at 490 nm, where the HC-RED 3 dyestuff has an absorbance peak. From Figure 8 it can be seen that with the monochromatic light a much better differ- entiation between the undyed and the dyed parts of the fiber cross section is achieved. IHC- RED 31 lOO z ! 50 HAIR DIAMETER in pm I lOO A: NORMAL LIGHT B= MONOCHROMATIC LIGHT (•90nm} Figure 8. Transmission profile curves of a cross section from HC-RED 3 dyed hair measured with normal light (curve A) and at 490 nm (curve B).

DYE DISTRIBUTION IN HAIR CROSS SECTIONS 69 The absorbance value Tmin, where the highest concentration of dyestuff is found, is almost two times higher in the case of the monochromatic than for the normal light measurement. If maximum absorbance values of the dyestuff are known, it is therefore of advantage to carry out the photometric measurements with monochromatic light. If the maximum absorbance value of the direct dyestuff is not known or in the case where measurements are going to be carried out on oxidative dyestuffs which only develop color inside the fiber structure, the UMSP itself can determine the specific wavelengths of maximum absorbance with the help of the LAMBDA-SCAN program. In this case it is advisable to carry out this premeasurement on a cross-sectional area with a high dyestuff concen- tration. INFLUENCE OF DYEING TIME ON THE DYE DISTRIBUTION IN HAIR FIBER CROSS SECTIONS To outline the possibilities of the Scanning Photometer Microscope, dyeings on hair tresses with HC-RED 3 dyestuff were carried out using different times of treatment, i.e. 5 min., 15 min and 30 min., respectively. The dyeings were performed under practical conditions. After rinsing and drying, the cross sections were produced as de- scribed in "Preparation of Cross Section" above and the photometric measurements carled out as described in "Microphotometric Examination of Dyed Hair Fiber Cross Sections" above. In Figure 9 the "master" transmission profile curves for the three dyeing times are IHC - REO •1 lOO 50- I I I I o 20 /,o 60 80 HAIR DIAMETER in pm 5 MIN • • 15 MIN ...... 30MIN Figure 9. "Master" transmission profile curves of cross sections from hair which has been dyed with HC-RED 3 for 5 min., 15 min., and 30 min.