opinion: Mr. Justice Douglas
  delivered the opinion of the Court.
  An inspector of a division of the. Colorado Department of Health entered the outdoor premises of respondent without its knowledge or consent. It was daylight and the inspector entered the yard to make a Ringelmann test of plumes of smoke being emitted from' respondent’s chimneys. Since that time Colorado has. adopted a requirement for a seareh warrant for. violations of air quality standards. At the time of the instant inspection the state law required nó warrant and none was sought. Indeed, the. inspector entered no part of respondent’s plant to make the inspection.
  A federal Act under the administration of the 'Environmental Protection Agency (EPA) sets certain air quality . standards, 81 Stat. 485, 42 U. S. C. § 1857 et seg. The States have the primary responsibility to assure the maintenance of air. quality standards, 42 U. S. C. § 1857c-2 (a). Yet if the EPA has approved'or promulgated “an applicable implementation” plan, a State may not adopt Or enforce a “less stringent” one, 42 U. S. C. §4857d-1. - There is no conflict between a federal standard and state. action, the sole question presented being whether Colorado has violated federal constitutional procedures in making the inspection in the manner described..
  Respondent requested a hearing before Colorado’s Air Pollution Variance Board. The Board held a hearing and found that respondent’s, emissions were in violation of the state Act. While the test challenged here was made on June 4, 1969, the Board after noting that Colorado’s Health Department had been in conference with respondent “in regard to its air pollution violations since September, 1967,” after approving the readings made by the field inspector on the day in question, and after holding that tests submitted in rebuttal by respondent were not acceptable, denied a variance and entered a ceasé-and-desist order. Respondent sought review in the District Court for Weld County which set aside the Board’s decision. The Colorado Court of Appeals affirmed, 610 P. 2d 907; and the Supreme Court denied certiorari.
  The petition for certiorari which we granted, 414 U. S. 1166, raised three questions, presenting in differing postures questions under the Fourth Amendment, made applicable to the . States by the Fourteenth. Mapp v. Ohio, 367 U. S. 643.
  The main, thrust of the opinion of the Court of Appeals is directed at the Fourth Amendment problem. It held that, under Camara v. Municipal Court, 387 U. S. 523, and See v. City of Seattle, 387 U. S. 541, the act of conducting the tests on the premises of respondent without either a warrant or .the cóñsent’of anyone from respondent constituted an unreasonable search within the meaning of the Fourth Amendment. We adhere to Camara and See btit we think they are not applicable here. The field inspector' did not enter the plant or offices.- - He was not inspecting stacks, boilers; scrubbers, flues, grates, or furnaces; nor was his inspection related to respondent’s files or papers. He had sighted what anyone in the city who was near the plant could see in the sky — plumes of smoke. The Court in Hester v. United States, 265 U. S. 57, 59, speaking through Mr. Justice Holmes, refused to extend the Fourth Amendment to sights seen in “the . open fields.” The field inspector was on respondent’s property but we are not advised that he was on premises from which the public was excluded. Under the Noise Control Act of 1972, 86 Stat. 1234, 42 U. S. C. § 4901 et.seq. (1970 ed., Supp. II), an inspector may enter a railroad right-of-way to determine whether noise standards are being violated. The .invasion of privacy in either that case or the present one, if it can be said to exist, is abstract and theoretical. The EPA regm lation for conducting an opacity test requires the inspector to stand at a distance equivalent to approximately two stack heights away but not more than & quarter of a mile from the base of the stack with the sun to his back from a vantage point perpendicular to the plume; and he must take at least 25 readings, recording the data at 15-to 30-second intervals. Depending upon the layout of the-plant, the inspector may operate within or without the premises but in either case he is well within the “open fields” exception to-the Fourth Amendment'approved.in Hester.
  The Court of Appeals went on to say that since respondent was not aware that the inspector had been on the premises until the cease-and-desist notice, the hearing it received “lacked the fundamental elements of due process of law-, since.the secret nature of the investigation foreclosed Western from putting on any rebuttal .evidence.”
  Whether the Court referred to Colorado “due process” or ■ Fourteenth Amendmept “due process” is not clear. If it is the former, the question is a matter of state law beyond our purview. Since we are unsure of the grounds of that ruling we intimate no opinion on that issue. But on our remand we leave open that.and any other, questions that may be lurking' in the case.
  Reversed and remanded.
  This test is prescribed by Colo. Rev. Stat. Ann. § 66-29-5 (Supp. 1967). It requires a'trained inspector to stand _iií a position where he' has an unobstructed view of the smoke plume, observe the smoke, and .rate it according to the opacity scale of the Ringelmann chart..- The person using the- chart matches the color and density of the sinoke • plume with • the numbered example on the chart. The Ringelmann test is generally sanctioned for use in measuring air pollution. See cases collected in Portland v. Fry Roofing Co., 3 Ore. App. 352, 355-358, 472 P. 2d 826, 827-829.
  Colo. Rev. Stat. Ann. § 66-29-8 (2) (d) (Supp. 1969).
  The Air Pollution Variance Board, after the Division of Administration; Colorado Department of Health, had issued a cease- .and-desist order; received a request from respondent for a hearing which was granted and held September 11,1969.
  EPA studies indicate that tests of stacks are expensive and may . require 300 man-hours of skilled work. 39 Fed. Reg. 9309. And see Schulze; The Economics of Environmental Quality Measurement, 23 J. Air Poll Control Assn. 671. (1973); 40 CFR § 60.85, Method 9.
  510 P. 2d, at 909.
  In the District Court’s opinion it is said that one challenge to the hearing before the Variance Board "was “whether or not due • process of law ánd equal protection of -the law contrary to the 14th Amendment of the Constitution of the United States and Section 25, Article 2 of the Constitution of the State of Colorado was denied” by the Board. App. 136.
  See California v. Krivda, 409 U. S. 33; Department of Mental Hygiene v. Kirchner, 380 U. S. 194; Minnesota v. National Tea Co., 309 U. S. 551,
choices: cert appeal bail certification docketing fee rehearing or restored to calendar for reargument injunction mandamus original prohibition stay writ of error writ of habeas corpus unspecified, other