In April, Tesla CEO Elon Musk had harsh words for the honeycomb-looking sensors — known as lidar — that companies like Alphabet's Waymo use to give their self-driving cars an understanding of the road and what's on it.

"Lidar is a fool's errand," Musk said. "Anyone relying on lidar is doomed."

On Wednesday at an event at Google's developer's conference, I/O, Waymo executives said that Musk's decision to leave out lidar from Tesla's autonomous approach was "very risky."

said that Musk's decision to leave out lidar from Tesla's autonomous approach was "very risky." The Waymo execs also said that the costs of lidar's are significantly dropping, downplaying Musk's argument about the sensor's high price tag.

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In April, Tesla CEO Elon Musk had harsh words for the sensor technology that most self-driving cars companies rely on, but on Wednesday executives at Alphabet's Waymo,who pioneered that tech defended their approach.

Musk's attack was centered on the honeycomb-looking sensors — known as lidar — that Waymo and other companies like Uber, Ford, and GM Cruise use in conjunction with cameras to give their self-driving cars an understanding of the road and what's on it.

Musk said Tesla's autonomous approach will solely rely on built-in cameras. "lidar is a fool's errand," Musk said. "Anyone relying on lidar is doomed."

Read more: Elon Musk says Tesla will have 1 million robo-taxis on the road next year, and some people think the claim is so unrealistic that he's being compared to PT Barnum

On Wednesday at an event for Google's developer's conference conference, I/O, Waymo's Principal Scientist Drago Anguelov said that Musk's decision to use only cameras and leave out lidar was "very risky." Lidar continuously bounces light waves around to help the car measure distances and "see" what's around it.

"You can imagine doing [autonomous] driving just with cameras, but you would need the best camera systems to really handle it," Anguelov said. "So that's a very big bet that you can achieve it. And it's very, very risky, and it's not necessary."

Anguelov said lidar sensors help Waymo create a safer experience for the passenger.

"We have much richer data and much more accurate. It's easier to build the right simulation environments," he said. Llidar also helps cars determine how other vehicles and objects in the road are interacting with each other and "all of this is considerably harder if you just use cameras, and more limiting," he said.

Dmitri Dolgov, Waymo's CTO and VP of engineering, echoed the message of safety that its sensors, alongside its cameras, bring to its autonomous approach.

"It's not one or the other — it's both," Dolgov said. "So it's all about taking the best of both worlds and combining them in an intelligible way to have the most capable and the safest system that you can have."

These engineers also tried putting to rest Musk's accusations that the high-price for lidars was one of its major downsides. Dolgov admitted that early versions of the lidar were "hugely expensive," but that the high costs were no different to most new technology when it's first developed.

"There's nothing fundamentally expensive about lidars," Dolgov said. "We've reduced the price from the first generation to [the current] generation of lidars by a huge margin. And you can imagine what the savings will be like as we scale."

In March, Waymo announced that it would start selling its lidar sensors to customers who don't compete with the company's autonomous ride-sharing services. The additional channel could help Waymo increase economies of scale through greater production, eventually bringing down the price of making the component for its own cars even further.

In terms of when the company will expand its autonomous ride-sharing service — Waymo One — outside of its initial testing market in the Phoenix area, Waymo execs were tight-lipped on a timeline.

"We have a roadmap and some plans to expand to locations beyond Phoenix," Dolgov said. "But as always, our deployments will be gated by safety."

On the flip side, Musk — who's ride-hailing plan lacked many important details — said Tesla would have 1 million autonomous taxis on the road "next year."