【文章标题】：荧光灯（不）长耳朵
【文章正文】： I never mentioned it publicly, but early in my career, I did a part-time stint in technical surveillance countermeasures (TSCM) — a fancy term for sweeping office environments in search of listening devices and other unauthorized spy gear. 我从未公开提及此事，但职业生涯早期曾兼职从事过技术反监视工作（TSCM）——这个花哨术语指的是排查办公环境中窃听装置等非法监控设备的工作。
In practice, the job entailed getting several certifications, hauling around a bunch of costly suitcases, and above all, spending some time with ex-spooks, listening to stories that would make James Bond blush. 实际工作中需要考取多项认证，随身携带几个昂贵的设备箱，最重要的是与退役特工共处，听他们讲述让詹姆斯·邦德都脸红的传奇故事。
The discipline is rather hush-hush, so you never know what’s real. One of the more striking claims I remember from the training was that fluorescent lamps could be used to passively eavesdrop on conversations in the room. 这个领域相当隐秘，真伪难辨。培训中最令人震惊的说法是荧光灯可被动窃听室内对话——这有一定道理：灯管内充满发光气体，理论上声波通过这种介质传播会产生能被远程捕捉的微妙亮度波动。
To be clear, long-distance optical audio pickup is real: if you shine a laser at a pane of glass or other reflective surface, sound-induced vibrations can be picked up by measuring the angle of the reflected beam; in favorable conditions, this supposedly works at distances in excess of 100 m (330 ft). 需说明的是，远距离光学拾音确实存在：用激光照射玻璃等反射面时，通过测量反射光束角度可捕捉声波振动；理想条件下据说有效距离超100米。
Far less practically, a Black Hat presentation in 2020 demonstrated the ability to passively recover audio by placing a beefy speaker 1 cm away from a dangling lightbulb and then watching the motion of the lightbulb via a telescope from about 25 m (80 ft). 更不切实际的是，2020年黑帽大会上有人演示：将大功率扬声器置于悬垂灯泡1厘米处，再通过25米外望远镜观察灯泡运动来被动还原声音。
The lightbulb research made it to The Wall Street Journal; this blog post won’t. Never mind that, though: the claim about fluorescent lamps seems physically plausible, but is true? 灯泡研究登上了《华尔街日报》，这篇博客则不会。不过关键在于：荧光灯窃听理论看似合理，但属实吗？细想有几个疑点：首先灯管内气体压强仅为大气压1/200，近乎真空——并非声波良导体；其次发光气体释放的紫外线需通过管内不透明荧光粉层转化为可见光，该涂层具有强余辉效应，难道不会掩盖任何瞬时局部亮度变化？……
After two short decades, I couldn’t take it anymore and decided to run a test. My initial plan was to tape a photodiode directly to the tube, connect the sensor into a low-noise amplifier, and then view the resulting waveform on an oscilloscope. 二十年后，我终难耐好奇决定实验。最初方案是将光电二极管直接贴在灯管上，传感器接入低噪放大器，再用示波器观察波形。
But that seemed like an overkill, so I eventually opted for a simpler approach: I placed the lamp next to a high-intensity sound source — a 200 W audio system hooked up to a signal generator and cranked all the way up — and then took a series of high-speed, up-close photos with a shutter of 1/8000 s. 但这似乎小题大做，最终选择更简单方法：将荧光灯置于200瓦音响系统旁（连接信号发生器并调至最大音量），用1/8000秒快门进行高速近距连拍——若顶级相机拍摄的14位原始图像中都看不到强声波造成的可见异常，实际可行性基本为零。
But first, I needed to power the tube. Traditional fluorescent lamps rely on thermionic emission to get going: there’s a pair of terminals on each end that connects to an internal heater coil. 首先需点亮灯管。传统荧光灯依赖热电子发射：两端各有一组连接内部加热线圈的端子。
Once the coil is heated to a glow, it becomes easier for thermally-excited electrons to dart off into the void in response to an externally-applied electromotive force. In this respect, the device is similar to a vacuum tube. 线圈加热至炽热后，热激发电子更易在外加电动势作用下跃入真空——这方面类似电子管。
For the 9” tube I purchased, the heater needed a current of about 200 mA at 16 V. I opted for DC operation to minimize AC-induced flicker, so it was sufficient to heat just the negative side. 我购买的9英寸灯管需16V/200mA电流加热。为减少交流电闪烁选用直流供电，仅需加热负极。
With that done, the terminals on each end would be shorted and a voltage of roughly 70-80 V would be applied across the device. This voltage is enough for plasma to form; from that point on, the current must be capped to about 180 mA at ~35 V. 随后短接两端端子并施加70-80V电压，足以形成等离子体；此后需将电流限制在35V/180mA左右。
Here’s a quick video showing the process of manually starting the lamp: 以下是手动点亮灯管的快进视频：
I’ll spare you the dozens of rapid-shutter photos I’ve taken while playing back different audio frequencies: they show nothing at all. 不同音频频率下的数十张高速快门照片均无异常，具体可观看右侧扬声器阵列全频段扫描视频：
I really wanted to believe the claim. Maybe someone else can still “prove” it; pump the volume up even higher, use a larger tube, take absurdly precise measurements. But in terms of a practical attack, I think the myth is busted. Sorry, Mr. Bond? 我本愿相信这个说法。或许有人能用更大音量、更大灯管、超高精度测量”证实”它。但作为实际攻击手段，我认为这个传说已被粉碎。抱歉了，邦德先生？