'Light Emitting Diodes 2e'

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from Light Emitting Diodes 2e, E. Fred Schubert, Cambridge {pgs. 14-15}

"… early application of LEDs was the numeric display in pocket calculators. Figure 1.9 shows two programmable calculators of the mid 1970s, the Texas Instruments Corporation’s SR-56 and the Hewlett-Packard Corporation’s HP-67. Both used red GaAsP LEDs in the seven segment numeric display. All calculators using LED displays shared a significant problem: the display could not be read under bright outdoor conditions because the light emitted by the LEDs was simply too dim; furthermore, the power consumption of LED displays was high so that the rechargeable battery running the calculator had to be recharged on a regular basis. Digital wrist watches using LEDs shared the same problem. Liquid crystal displays (LCDs), introduced at the end of the 1970s, had much lower power requirements. Consequently, LCDs totally replaced LED displays in calculators and watches by the beginning of the 1980s.
Fig. 1.9. Programmable calculators Model SR-56 of the Texas Instruments Corporation and Model HP-67 of the Hewlett-Packard Corporation, both first manufactured in 1976. Seven-segment numeric characters composed of GaAsP LEDs were used in the display. The SR-56 came with a 'huge' program memory of 100 steps. The HP-67 came with a magnetic card reader and had several freely programmable keys."

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ps: color, subdued & bold emphasis mine.