Translucent dentin can be either carious or sclerotic dentin

It is believed that translucent dentin under light microscopy represents sclerotic dentin. The most common light microscopy techniques used to detect translucent dentin are bright field transmitted light microscopy and stereo microscopy with episcopic illumination (reflected light). In addition, it has been proposed that sclerotic dentin presents itself as an area with higher brightness under polarizing microscopy (with dark background) [Bjorndal L, Thylstrup A (1995). A structural analysis of approximal enamel caries lesions and subjacent dentin reactions. Eur J Oral Sci 103: 25-31]. 

 The idea that translucent dentin is sclerotic is highly influential in many subjects, to quote some:
i- measurements of the depth of caries lesions;
ii- the determination of the first dentin reaction to caries (for shallow enamel caries lesions);
iii- the relationship between visual features of the tooth surface (including visual scoring systems for screening of caries lesions) and lesion severity (based on lesion depth).
  The main reason is that sclerotic dentin is hypermineralization of dentinal tubules. Sclerotic dentin does not represent carious demineralization. From the tooth surface to the junction between dentin and pulp (soft tissue with blood vessels and nerves), carious dentin is located closer to the tooth surface than sclerotic dentin. Thus, when measuring the depth of a carious lesion, sclerotic dentin outlines the innermost part of the carious dentin. And it is believed that translucent dentin can be reasonably considered as sclerotic dentin (Kidd & Fejerskov, Journal of Dental Research, 83(suppl.01): C35-C38, 2004).
  There are, however, reports indicating that translucent dentin can be carious dentin. The earliest reports can be found in:

11) Applebaum E, Hollander F, Bodecker C (1933). Normal and Pathological Variations in Calcification of Teeth as Shown by the Use of Soft X-rays. Dent Cosmos 75: 1097–1105 [compare Fig.1 (transmitted light microscopy image showing translucent dentin) on page 1098 with fig. 2 (micro radiographic image showing demineralized dentin related to translucent dentin) on page 1099, both from the same ground section of the same tooth; full text found at http://quod.lib.umich.edu/d/dencos/0527912.0075.001/1201:774?rgn=main;view=image  or search on http://quod.lib.umich.edu/d/dencos/];

22) Applebaum E (1935). Tissue changes in caries. Dent Cosmos 77: 931–941 (compare fig. 1 with fig. 2, both on page 933; full text found at http://quod.lib.umich.edu/d/dencos/0527912.0077.001/1310:750?rgn=main;view=image);

33) Gottlieb B, Diamond M., Applebaum E (1946). The caries problem. Am J Orthod Dent Surg 32: 365–379;

44) Gottlieb B (1946). Dental caries. Philadelphia: Lea & Febiger (book).

      The above cited reference provide evidence that translucent dentin can be either carious or sclerotic. The current view in the scientific field of cardiology and oral histology is that translucent dentin is the same as sclerotic. A change is needed on how translucent dentin is interpreted. Micro radiography is the most suitable technique to detect demineralization and sclerosis.

      Below, figs. 1 and 2 from Applebaum et al (1935) are shown.