Spectral absorption characteristics

Dye Developers. In addition to having suitable diffusion properties, dye developers must be stable and inert in the negative before processing. After completion of the process, the dye developer deposited in the image-receiving layer must have suitable spectral absorption characteristics and stabiUty to light. 

Based on some early work in the 1930 s, the assumption has been that the chromophores of vision are based on retinol or retinal coupled to a protein, opsin, in such a way as to form a chromophore. The variation in the location and possibly the style of couphng has been assumed to account for the actual spectral absorption of the material. After massive efforts, this premise has still not been demonstrated. However, it has caused a great deal of concentration on retinol and retinal as the fundamental structure of the chromophores. To this day, no laboratory experiments have shown either of these two retinoids exhibit a spectral absorption characteristic matching those of vision, human or otherwise. The retinols are neither chromophores nor analogs of the chromophores of vision. 

The absorption cross section associated with the above quantum-mechanical resonance is strongest for a straight molecular structure. This leads to a maximum in the spectral absorption characteristic for a dipole molecule when in the all-trans form. For other configurations, the structure exhibits several lesser structural resonances along different axes that lead to shorter wavelength spectral absorption peaks. This effect was studied extensively by Scheibe33 and an excellent figure in Venkataraman describes it. 

In the case of vision, the chromophores of vision are usually explored in the laboratory while in solution. However, in-vivo, they are not in solution but in the liquid crystalline state. This state of matter causes the chromophores to exhibit an entirely separate spectral absorption characteristic. 

Special circumstances must exist to measure the spectral absorption characteristics of the Rhodonines in-vitro in the laboratory (where in-vitro implies the separation of the chromophores from their connection to the neurons) under steady-state conditions. The material must be in the liquid crystalline state and an artificial method of de-excitation must be provided. 

As indicated in Section 5.4.5, it is not appropriate to consider the observed spectral absorption characteristic of a single chromophore as a single function and speak of the half-amplitude points as describing the waveform. The peak wavelength and the two half-amplitude points can be used for less critical work. However, the correct description of the waveform requires that the waveform on each side of the pseudo-peak be plotted as an exponential function and the wavelength specified at which this function is equal to 1/e of its peak value. These two 1/e values properly describe the measured spectral response. 

They all exhibit the same spectral absorption characteristic when in the dilute (less than 10 2 M) liquid state. 

As a result of this enhancement, when the spectral absorption characteristics of the four Rhodonines are plotted on one graph, the resulting family of curves closely resembles [Figure 5.5.9-1] from Mees James. 

The ancillary peaks related to the Bezold-Brucke and the Purkinje Effects are particularly relevant to determining the precise amount of crossover between the spectral absorption characteristics. A putative peak near 390 nm in the aphakic human eye would also be very useful in evaluating these parameters. See Section 17.2.4. 

In those instances where the sample has undergone extensive chemical reaction such as, for example, oxidation during bleaching, the spectral absorption characteristics of the lignin may have been altered to such a degree as to make measurement of absorbance at 280 nm meaningless. Under these circumstances neither the acetyl bromide method nor any other spectrophoto-metric method represents a viable analytical approach. 

A discussion of photostability is not as straightforward as that of thermal stability due to the complexity of photoexposure. Correlation between devices of the same type, either in the same laboratory or at different locations, can be very good if they are operated under the same test conditions. Test results between devices with different sources of radiation (e.g., xenon arc and fluorescent tube) will vary according to the spectral absorption characteristics of the product being tested. Thus it is possible that some products may decompose at an equal rate in systems having different types of irradiation sources however, other products may react quite differently. Although the proposed test is reasonably simple to conduct, there are some practical problems not definitively resolved (e.g., selection of adequate irradiation source and calibration). This chapter will focus on practical problems related to the use of the current guideline. 

Routine u.v. studies of the effects of sulphur-containing groups on the spectral absorption characteristics of substituted benzenes (references are collected at the end of this section) continue along familiar lines (see Vol. 5, p. 4). A model for the thioindigo chromophore, i.e, MeCOC(SMe)=C(SMe)COMe, has been shown by X-ray studies to be non-planar, as a result of repulsion between methyl groups. 

Fig. 10.10 Spectral absorption characteristic of water films with thicknesses 2 and 10 im. Illustration courtesy of Heraeus Noblelight Ltd.

Correlation between devices of the same type, either in the same laboratory or at different locations, can be very good if they are operated under the same test conditions. The results between devices with different sources of radiation (xenon lamp or carbon arc) will vary according to the spectral emission characteristics of the sources and spectral absorption characteristics... 

Considerable variation in both the aryl and alkyl groups in the above structures can be tolerated while maintaining the photoactivity of these salts. Through the introduction of substituents and structural modifications, it is possible to manipulate both the spectral absorption characteristics and the photosensitivity of both types of photoinitiators. In addition to those sulfonium salts represented by structures III and IV, several related compounds such as alkylarylphenacylsulfonium salts and dialkyl-2-hydroxyphenylsulfonium salts have also been observed to be excellent cationic photoinitiators... 

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