The 99.2% That Names the Real Design Lever

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

The 99.2% That Names the Real Design Lever

A glucose-monitoring smart lens takes 216 readings and 36 NFC bursts across an 18-hour wear, and the chapter totals the whole day at just 32.7 mJ. Its key insight is startling: the parts a designer instinctively optimises — sensing and the radio — barely register, because MCU sleep current alone is 99.2% of the budget. So what does that 99.2% figure actually name as the real design lever?

Companion to the chapter Smart Contact Lenses — every number here comes from that chapter.

Ada: The power-budget worked example ends on a striking claim: MCU sleep current is 99.2% of the lens’s daily energy. If that holds, it tells the designer exactly where to spend effort. Let me total the day from the chapter’s own component draws – 216 readings and 36 NFC bursts across an 18-hour wear.

• Energy per reading: 2 x 0.15 + 12 x 0.05 + 1 x 0.01 = 0.30 + 0.60 + 0.01 = 0.91 uJ; sensing total 216 x 0.91 = 196.56 uJ.
• Energy per burst: 30 x 0.05 + 12 x 0.02 = 1.50 + 0.24 = 1.74 uJ; transmission total 36 x 1.74 = 62.64 uJ.
• Sleep energy: 0.5 x 18 x 3600 = 32,400 uJ.
• Day total: 196.56 + 62.64 + 32,400 = 32,659.2 uJ, i.e. 32.7 mJ, at an average of 32,659.2 / 64,800 = 0.50 uW.
• Sleep share: 32,400 / 32,659.2 = 0.992063, which rounds to 99.2%.

The whole example checks out. The design meaning is that sensing and radio – the parts a designer instinctively optimises – together sit under 0.8% of the budget, so halving the 150 ms sensor window saves almost nothing, while a 10x lower sleep current cuts total energy nearly 10x; the arithmetic is what redirects effort from the visible active path to the invisible sleep-current leak.

Every number above is taken from the chapter’s own material and re-derived step by step.